rook endgame or pawn endgame
Study Pawn endgames first. Then Rook endgames.
... and that will cover about three-quarters of the end-games that you'll likely face.
Two of the most important concepts are :
For Pawn endgames - the Opposition.
Opposition - Chess Terms - Chess.com
And for Rook endgames, the Lucena position.
Lucena Position - Chess Terms - Chess.com