Many Git commands take revision parameters as arguments. Depending on the command, they denote a specific commit or, for commands which walk the revision graph (such as linkgit:git-log[1]), all commits which are reachable from that commit. For commands that walk the revision graph one can also specify a range of revisions explicitly.
In addition, some Git commands (such as linkgit:git-show[1] and linkgit:git-push[1]) can also take revision parameters which denote other objects than commits, e.g. blobs ("files") or trees ("directories of files").