IP (Ingress Protection) code is an international code for classifiying protection of electrical appliances. The electrical enclosures are protected against mechanical contact and water dependending on their purpose an nature of use. The level of protection is marked by the two numbers after IP code:

Solid particle protection
0          none
1          against solid particles larger than 50 mm
2          against solid particles larger than 12 mm
3          against solid particles larger than 2,5 mm
4          against solid particles larger than 1 mm
5          against Ingress of dust with no harmful effect
6          complete protection against dust

Liquid ingress protection
0          none
1          against dripping water (vertically falling drops)
2          against dripping water when tilted at 15°
3          against spraying water at any angle up to 60°
4          against splashing water from any direction
5          against low pressure water jets
6          against powerful water jets and short term of immersion in water
7          against immersion in water up to 30 minutes
8          can be used continuous immersion in water

The three most common codes in interior design:
IP20 – simple interior lamps
IP44 – lamps can be placed in bathrooms (outside of shower cabin and bath!)
IP68 – lighting can be placed in swimming pools