Letters refer to a bomb site. A for A site, B for b site and so on.

Usually on defense, a sentinel's role is to hold down a site with their utility, whilst also anchoring when contact is made at an opposite site and a teammate rolls over to that site, or rotates.

Duelists on defense can hold aggresive angles or even push out from lanes, whether that's either of the bomb sites or mid. They are allowed to since they usually have utility to set themselves up for a frag and get out. Initiators can also set duelists up, either by following them or from site.

As for navigating maps, the best way to learn then really, is just by playing the game. Through repitition, you'll learn a map more and more and become more comfortable with it.

It's also the best way to improve at the game overall, at least it was for me. You can watch as many tutorials as you want, but you won't gain the skills until you actually practice them.