IP (or "Ingress Protection") ratings are defined in international standard EN 60529 (British BS EN 60529:1992, European IEC 60509:1989). They are used to define levels of sealing effectiveness against intrusion from foreign bodies (tools, dirt etc) and moisture.
The first digit is the degrees of protection against solid foreign objects.
The second digit is protection from moisture ingress.
Example: IP 67 (dust tight, protected against the effects of temporary immersion in water).