commit is the command you give in an SQL session, when you want to save the changes you made to the database. If you make a change, all other users that use the database will not see the changes until you commit them. Usually when you end an SQL session, a commit is automatically performed.
On the other hand, if you made changes, and want to undo them, you can use the rollback command, which rolls back all the changes you made since the last commit.