i keep seeing this term thrown around, and i'm just lost. not sure if its just a dumb question.
but what exactly does min-max mean?

basically what its says min-max, depends on the sentence used with it.
typically on LO it means minimising and maximising squads.
For example in DE a good example of min & max squads is having 6x5 man warrior squads with a lance in each squad, 6 raider's with a lance, maximising all troop slots, getting the most lances you can out of it and minimising squad size at 5 warriors the lowest you can use.

Another common usage is minimising your weaknesses while maximising your strengths, although in a... cheesy fashion, IE putting holofields and spirit stones on a falcon and making sure it moves 6"+ every turn.

I thought it ment minimise squad size but maximise firepower upgrades (like having Las/plas units of 6 marines).

My interpretation is "Minimun effort - Maximun Gain". That means building a list with the maximun firepower with the minimun cost.

Odd enough, I discovered that for others means "minimazing the maximun possible loss".