“To improve at chess you should in the first instance study the endgame.” José Raúl Capablanca
The endgame, as the name suggests, is the last stage of the game. Unfortunately, there’s no universal definition of the start of the endgame. The most used definition states that the endgame starts when there are few pieces left on the board. Sometimes, people define the endgame as “when the queens are off the board”, and while this definition is practical, it doesn’t always work (king and queens endgame are actually really complicated).
The study of the endgame is crucial to the development of your understanding of the game. It’s also a phase of the game where general principles are not enough, there are rules you need to follow in order to calculate precisely your move.