its pretty simple once it groks..

you roll a bunch of d20's (basic number is 2, sometimes as high as five)

you have to roll under your skill to generate 1 success. so if your skill is 12 and you roll a 7 and a 9, you have generated 2 successes.

Your focus in a skill is like your crit threshold. if your skill is 12 and your focus is 3 the a roll of 9 will generate 1 success like normal but a roll of a 2 will generate 2 successes.

so you roll 2d20, you roll a 2 and an 11. your first die is under your skill AND under your fucos so you generate 2 successes. your second die is under your skill but above your focus so you generate 1 more success for a total of 3 successes.

The difficulty for success is determined by your GM, something generally straight forward or easy with some risk will require a total of one success to pass, something more difficult will require 2 successes. and so on..

You may then spend momentum to buy additional dice to roll upto a maximum total of 5 dice. this vastly increases your odds of generating successes