IP stands for Ingress Protection and is defined in IEC 60529.

It describes the degrees of protection provided by enclosures

This standard describes a system for classifying the degrees of protection provided by the enclosures of electrical equipment. Developed by the European Committee for Electrotechnical Standardization (CENELEC), these standards are designed to numerically rate an electrical product on the level of protection its enclosure provides. By assigning different number codes, the product's degree of protection can be identified quickly and easily. In the code IP 54, for example, IP identifies this standard, the 5 describes the level of protection from solid objects, and 4 describes the level of protection from liquids.

First digit describes the level of protection from solid objects

0
 Non-protected
1
 Protected against solid foreign objects 50 mm diameter and larger
2
 Protected against solid foreign objects 12.5 mm diameter and larger
3
 Protected against solid foreign objects 2.5 mm diameter and larger
4
 Protected against solid foreign objects 1.0 mm diameter and larger
5
 Dust Protected
6
 Dust-tight

Second digit describes the level of protection from liquids

0
 Non-protected
1
 Protected against vertically falling drops of water
2
 Protected against direct sprays up to 15° from the vertical
3
 Protected against direct sprays up to 60°from the vertical
4
 Protected against sprays from all directions - limited ingress permitted
5
 Protected against jet-water
6
 Protected against strong jet-water
7
 Protected against the effects of temporary immersion in water
8
 Protected against long periods of immersion under pressure