Tagging is the act of marking a specific revision in source control with an identifier for future reference.
A tag in source control is essentially an immutable branch, although in practice, many source control systems do not enforce this.
The purpose behind tagging is to make it possible in the future to reference the contents of the repository as it was at a specific point in time. For example, a software release may be tagged. Development continues on the trunk, but the tag is still available to review in case a bug is reported against that version.