A revision number is a small integer.
It is a strictly local convenience identifier for a revision. It can be useful as it is shorter to type than the 40-digit hexadecimal string that uniquely identifies a revision.
Revision numbers referring to changesets are very likely to be different in another copy of a repository. Do not use them to talk about changesets with other people. Use the changeset ID instead.
The Mercurial command-line client provides much more detailed information on Revision Sets. Just run:
$ hg help revsets
An empty repository (with no changesets) has a revision number -1, which corresponds to the nullid nodeid. For example:
$ hg log $ hg status $ hg head changeset: -1:000000000000 tag: tip user: date: Thu Jan 01 00:00:00 1970 +0000