In the electrical world an IP rating is the rating given to a certain product to represent the quality of water and substance resistance. This is an extremely important aspect of electrical products, as if they are exposed to these elements for a prolonged period of time, then they will cease to work and can also cause injury.
The standards are set by the IP Code, or Ingress Protection Code, IEC standard 60529, (also known as the International Protection Code), and companies have to build their products to the specifications and standards, then send their products off for inspection and testing.
The IP rating is the given rating for resistance against alien substances and the rating is split in a way that will show the resistance from solid substances first, in a 1-6 rating with 6 being the best and then the second digit is the water resistance rating, which is given a 1-8 rating. It is worth noting that water resistance and being waterproof are different, water resistant is exactly what it says, a resistance, so that it cannot remain submerged indefinitely.