Clicking Save will record any entries or edits you have made to your timecard during that log-in. A best practice is to enter your leave throughout the month; you must click Save before you log out to record those entries. (If you ever enter leave time that you do not end up taking, remember to delete it by the end of the month.)
By clicking Approve, you are verifying that your entire timecard accurately reflects your use of leave time that month – either the amount and type of leave that you took, or the fact that you took none at all. You are required to approve your timecard every month, whether you took leave time or not.
So remember to Save your timecard throughout the month whenever you make entries or edits to it, then Approve it by the 1st of the following month.