This refers to the Ingress Protection or International Protection ratings which are an international standard. These standards define the levels of sealing effectiveness of electrical enclosures against intrusion from foreign bodies such as dirt and water.
The rating consists the letters IP followed by 2 digits. The first digit stands for the level of protection that the enclosure provides against solid bodies, the second digit describes the degree of protection of the equipment inside the enclosure against moisture.
Protection against solid bodies comes in a 0-6 rating with 0 being not protected, 3 being protected against solid objects greater than 2.5mm in diameter and 6 being dust tight.
Protection against moisture is in a 0-8 scale. A rating of 0 means it is not protected from moisture at all. A rating of 4 means it is protected against splashing, 5 means protection against water jets from any angle and 8 means protection against continuous submersion.
For example, if a light has IP rating of IP65, then it has a 6 rating for solid bodies and a 5 rating for moisture protection. On the international standard scale this means that it is the highest level of dust tight and is protected against water jets from any direction.