IP stands for ingress protection, a standard developed by the International Electrotechnical Commission. Originally, it defined how well industrial enclosures coped with dust and water. Today, many consumer devices are also included. The initials IP are followed by two digits. The first is a rating for particles, the second is for liquids.Full specifications are available online but to give you a rough idea: an IP54 case might allow in dust, but not enough to stop it from working. It would survive water splashes – like five minutes under a garden sprinkler. An IP65 case is completely sealed against dust, and would survive a good soaking from a hose – but not total immersion.