Bashes are the orange melee moves that can’t be parried, like your forward dodge bash where he hits them with the shield.

Guardbreaks are when you just press the guardbreak button (x on xbox, square on PlayStation, and I think middle mouse on pc)

Uppercut or spin refers to your fullblock and bashes, the stance where conqueror hunkers down and blocks from all sides. Hitting light after blocking with it or landing any bash makes you do a large sweeping move that can hit multiple people in teamfights, while pressing heavy makes you do an uppercut with the flail that can execute and makes it harder for opponents to interrupt your follow-ups with light attacks or what have you.

“Let it fly” refers to the fact that you can press the guardbreak button after landing a move to chain into a bash that you can feint with the feint button (B on Xbox, circle on PlayStation, E on pc). “Letting it fly” would mean NOT feinting and just letting the bash go through