The IP rating is an international standard that tells you how resistant a device is to solids and liquids. As applied to lights, the first digit defines protection from dust, and the second digit defines protection from water.
IP00 - no protection from objects, no protection from rain
IP01 - no protection from objects, protection from vertical light rain
IP02 - no protection from objects, protection from light rain
IP03 - no protection from objects, protection from rain and spraying
IP04 - no protection from objects, protection from rain, spraying and splashing
IP05 - no protection from objects, protection from rain, spraying, splashing and nozzle pressure
IP06 - no protection from objects, protection from all rain and sea conditions
IP07 - no protection from objects, protection from accidental submersion in water
IP08 - no protection from objects, protection from accidental deep submersion in water
IP10 - protection from objects > 50mm in size, no protection from rain
IP11 - protection from objects > 50mm in size, protection from vertical light rain
IP12 - protection from objects > 50mm in size, protection from light rain
IP13 - protection from objects > 50mm in size, protection from rain and spraying
IP14 - protection from objects > 50mm in size, protection from rain, spraying and splashing
IP15 - protection from objects > 50mm in size, protection from rain, spraying, splashing and nozzle pressure
IP16 - protection from objects > 50mm in size, protection from all rain and sea conditions
IP17 - protection from objects > 50mm in size, protection from accidental submersion in water
IP18 - protection from objects > 50mm in size, protection from accidental deep submersion in water
IP20 - protection from objects > 12.5mm in size, no protection from rain
IP21 - protection from objects > 12.5mm in size, protection from vertical light rainr
IP22 - protection from objects > 12.5mm in size, protection from light rain
IP23 - protection from objects > 12.5mm in size, protection from rain and spraying
IP24 - protection from objects > 12.5mm in size, protection from rain, spraying and splashing
IP25 - protection from objects > 12.5mm in size, protection from rain, spraying, splashing and nozzle pressure
IP26 - protection from objects > 12.5mm in size, protection from all rain and sea conditions
IP27 - protection from objects > 12.5mm in size, protection from accidental submersion in water
IP28 - protection from objects > 12.5mm in size, protection from accidental deep submersion in water
IP30 - protection from objects > 2.5mm in size, no protection from rain
IP31 - protection from objects > 2.5mm in size, protection from vertical light rain
IP32 - protection from objects > 2.5mm in size, protection from light rain
IP33 - protection from objects > 2.5mm in size, protection from rain and spraying
IP34 - protection from objects > 2.5mm in size, protection from rain, spraying and splashing
IP35 - protection from objects > 2.5mm in size, protection from rain, spraying, splashing and nozzle pressure
IP36 - protection from objects > 2.5mm in size, protection from all rain and sea conditions
IP37 - protection from objects > 2.5mm in size, protection from accidental submersion in water
IP38 - protection from objects > 2.5mm in size, protection from accidental deep submersion in water
IP40 - protection from objects > 1mm in size (eg wire), no protection from rain
IP41 - protection from objects > 1mm in size (eg wire), protection from vertical light rain
IP42 - protection from objects > 1mm in size (eg wire), protection from light rain
IP43 - protection from objects > 1mm in size (eg wire), protection from rain and spraying
IP44 - protection from objects > 1mm in size (eg wire), protection from rain, spraying and splashing
IP45 - protection from objects > 1mm in size (eg wire), protection from rain, spraying, splashing and nozzle pressure
IP46 - protection from objects > 1mm in size (eg wire), protection from all rain and sea conditions
IP47 - protection from objects > 1mm in size (eg wire), protection from accidental submersion in water
IP48 - protection from objects > 1mm in size (eg wire), protection from accidental deep submersion in water
IP50 - significant protection from dust, no protection from rain
IP51 - significant protection from dust, protection from vertical light rain
IP52 - significant protection from dust, protection from light rain
IP53 - significant protection from dust, protection from rain and spraying
IP54 - significant protection from dust, protection from rain, spraying and splashing
IP55 - significant protection from dust, protection from rain, spraying, splashing and nozzle pressure
IP56 - significant protection from dust, protection from all rain and sea conditions
IP57 - significant protection from dust, protection from accidental submersion in water
IP58 - significant protection from dust, protection from accidental deep submersion in water
IP60 - air-tight, no protection from rain
IP61 - air-tight, protection from vertical light rain
IP62 - air-tight, protection from light rain
IP63 - air-tight, protection from rain and spraying
IP64 - air-tight, protection from rain, spraying and splashing
IP65 - air-tight, protection from rain, spraying, splashing and nozzle pressure
IP66 - air-tight, protection from all rain and sea conditions
IP67 - air-tight, protection from accidental submersion in water
IP68 - air-tight, protection from accidental deep submersion in water
This video provides a light-hearted explanation of IP rating