What does COMMIT do ?

Commit basically refers to an action which makes all the changes permanant since the last commit.
commit is alos availabale for non-Sql transanctions, like a method  available with Installer class which finalises
installation after all installation steps have been carried out successfully

public override void Commit(System.Collections.IDictionary savedState)
{
    base.Commit(savedState);
}

so commit simplly is a mechanism to finalise what you have done and make changes permanant

COMMIT makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions transactions that start only after transaction is committed.