Just means the board has been "revised" since it's previous revision, think of it as a "version" like software.
The actual nature of the revision could be anything.
For example, a later revision may have a better (or different) integrated graphics chip, or more RAM slots.
Some changes may not even be obvious just by looking at the board compared to an earlier revision.
But whatever differences there are, there are no major changes to warrant giving the board an entirely different model number.