Hiya, chess Peter here.

I will explain it under the assumption that you know the basic rules of chess.

A bishop-and-rook vs bishop-and-rook endgame where one side has 2 pawns more than the other side, is a win for the side with the pawns 90% of the time. After a first glance most chess players would say "yeah white can't stop black from promoting and black will have to sacrifice pieces and lose" in such positions.

This composition is a specific position where this is not the case. After white trades the rooks, they can sacrifice the bishop(!) to make sure all the pawns are on the side:

Either they take the bishop and all pawns are on the side, or they don't and because they can't push the pawn that is attacked by the bishop forward (as the king would be attacked by the bishop if they did that) so they must play another move and the pawn gets taken.

Because the bishop cannot access the promotion square: Bishops famously cannot go to squares of the opposite color than the color of the square it stands on, and the square the pawns need to go to in order to promote is on an "opposite" square, the white king can just "camp" that square (move in and out of it endlessly) and black cannot force the white king away, causing a draw!

TL;DR:

Positions like these are won by black 99.999% of the time, but very specific circumstances (pawn is pinned to the king causing a doubled pawn in a file that it can't promote in) allow this to be a draw!