MLF (or, commonly, 'malo') is shorthand for Malolactic Fermentation. Which is confusing, because it actually isn't a fermentation - it's a conversion, as no alcohol is produced as a result of MLF.

Malolactic Fermentation is a process whereby Malic Acid (found in apples) is converted into Lactic Acid (found in dairy), by the addition or natural occurrence of a bacteria. Essentially the bacteria consumes harsh malic acids, converting them to more palatable lactic acids. Some white wines go through MLF, and nearly all red wines do. For white wines it's inclusion or exclusion is often a stylistic choice made by the wine maker (though sometimes it happens unprovoked). 

What does Malolactic Conversion smell and taste like? Butter (Diacetyl), cheese (kinda dank gross-but-still-nice cheese smell), and cream are your main MLF signifiers. Importantly (like lees) it gives wine body and a creamy texture.

It's not an all-or-nothing game, either - quite often winemakers will allow only a portion of a white wine to go through MLF, to give it a little bit of weight and texture without losing all important acidity and/or fresh fruit character. Certainly a winemaker wouldn't want to completely soften the acid in their wine. 

Also worth mentioning that 'buttery' is widely regarded as a fault in winemaking, and is indicative of heavy-handed 'malo' use. 

Want to know more about MLF? You're a huge nerd, but this is a good source.  

NB: In your day-to-day Aussie drinking, you're most likely to recognise it in Chardonnay - if you're chasing that MLF dream, that's where I'd start.