MLF is MaloLactic Fermentation. It is usually conducted at the end of primary fermentation by inoculating with MLB (Malolactic Bacteria). It's primary purpose is to remove malic acid which is a sour acid (think green apples). It's not recommended to conduct MLF on kits as the kit maker has already adjusted the malic acid.
If you ferment with raw red or frozen red grapes, you should plan to do it. There are specific requirements for conducting it which I won't go into here. Usually, MLF is only done on red wines, rarely on whites. Be sure to read more about it before you attempt it and again, DON'T TRY IT ON A KIT!
Fred