They use a variation on semantic versioning, which is pretty standard:

    major.minor.revision

revision is incremented when backwards compatible bug fixes are added

minor is incremented when backwards compatible features are added

major is incremented when backwards incompatible change is introduced.

All of them are integers, and the smaller ones are reset to 0 when a bigger one is incremented.

For versions 0.x.x (pre v1), backwards incompatible changes are allowed at any time.

There are more rules, see http://semver.org/