The first number describes protection against solid objects:

1 Objects larger than 50 mm cannot get in.
2 Objects larger than 12 mm cannot get in.
3 Objects larger than 2.5 mm cannot get in.
4 Objects larger than 1 mm cannot get in.
5 Protected against dust but small amounts of dust can get in (dust resistant).
6 Totally dust proof.

The second number describes protection from moisture:

1 Vertically falling drops of water cannot get in.
2 Water sprayed from up to 15° from vertical cannot get in.
3 Water sprayed from up to 60° from vertical cannot get in.
4 Water sprayed from any direction cannot get in (limited ingress permitted).
5 Low pressure water jets from any direction cannot get in (limited ingress permitted).
6 High pressure water jets from any direction cannot get in (limited ingress permitted).
7 Suitable for immersion in up to 1 m of water.
8 Suitable for prolonged immersion under pressure.