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 hat each of the numbers represent.

  Solids (1st number)   Liquids (2nd number)
0 No Protection 0 No Protection
1 Protected against objects > 50mm (hands) 1 Protection against dripping water or condensation
2 Protected against objects > 12mm (fingers) 2 Protection against water spray 15 degree from vertical
3 Protected against objects > 2.5mm (tools/wires) 3 Protection against water spray 60 degrees from vertical
4 Protected against objects > 1mm (small tools) 4 Protection against water spray from all directions
5 Protected against dust, limited ingress 5 Protection against low pressure jets of water
6 Totally protected against dust 6 Protected against high pressure water jets and heavy seas
7 N/A 7 Protection against the effects of immersion (6 inches to 3.3 feet)
8 N/A 8 Protected against immersion