IP stand for Ingress Protection. It is a rating that describes the protection a fitting has from intrusion of Solid and Liquid material.

The letters IP are always followed by 2 numbers, the first refers to intrusion by solids (1-6) and the second refers to intrusion by liquids (1-8).

For Example a Rating of IP44 would be used on an exterior fitting that may get rained on. Where as an IP68 rating would allow the fitting to be submerged into water, eg a pond light.
The chart below gives a detailed overview of what each of the numbers represent.