See also: How to register, upload, publish, and release a charm
In the context of a charm, a revision is a number that uniquely identifies the published charm.
The revision increases with every new version of the charm being published.
This can lead to situations of mismatch between the semantic version of a charm and its revision number. That is, whether the changes you make to the charm are for a semantically newer or older version, the revision number always goes up.
Charm revisions are not published for anybody else until you release them into a channel. Once you release them, though, users will be able to see them at charmhub.io/<charm/channel> or access them via juju info <charm> or juju deploy <charm>.