Literally, from the stronger [point]; it is used to introduce a statement that, assuming a previous statement is accepted as true, must be all the more true. When arguing a point, a fortiori means "all the more so."

If statement "A" is true, then a fortiori, i.e., all the more so, statement "B" must be true. If students can't (or won't) do twenty minutes of homework each night, a fortiori, they can't (or won't) do sixty minutes each night.