A rook is five points and a knight is three, so when a rook comes off for a knight, somebody has gained two. That trade happens so often that English gave it a name of its own instead of spelling it out every time.
Win the exchange: your rook, his bishop or knight, you two points to the good. Lose the exchange: the same thing the other way round. It is one of the few pieces of chess slang that is completely precise.
How it usually happens
Not by anyone offering it. Almost always by a tactic, and at beginner level almost always a knight fork.
7 
2 
e 
g The black king has to answer the check, and the rook on c8 is gone next move. Nobody blundered a piece, nobody hung a queen, and yet the game has changed. Two points, collected by a fork.
The other regular route is a rook that walks onto a square a bishop covers. A rook on d4 with an enemy bishop on g7 is not a rook on a good square, it is a rook waiting to be traded down.
What the two points feel like
They feel like nothing for about ten moves and then like everything. The side with the rook opens a file, gets to the seventh rank, attacks pawns from behind and picks them off one at a time. The side with the bishop or knight needs the position to stay closed, needs a strong square in the centre, and needs the pawns to stay on one wing.
So if you win the exchange, trade the other pieces and open a file. If you lose it, keep the position blocked and find your knight a home on a square no pawn can attack.
Giving it up on purpose
Strong players hand over the exchange more often than beginners expect, and it is not a sacrifice in the fireworks sense. The usual price list: two points for a knight that can never be dislodged from d5, or for the pawn in front of a castled king, or for a passed pawn on the sixth.
I would not go hunting for it at 1000 rating. Learn the plain version first, the one in the diagram, because winning the exchange twice in a session is a rating point machine. Count the trade the way you count everything else, five against three, and remember that the same arithmetic runs the rest of the board: see material and the difference between minor and major pieces.