Material refers to the quantity of pieces and pawns on the board.
Generally, the player with more material can gain control by forcing the opponent’s pieces into defensive positions and then using the extra material to create and attack new targets.
Aim to win material because the player with more pieces can usually gain control by using their extra forces to attack more targets than what the opponent can defend.
The material objective
The first objective: Aim to win material
Bobby Fischer on the “material objective”
The legendary world-champion, Bobby Fischer, strongly believed in the importance of what we will now call the “material objective”:
“Concentrate on material gains. Whatever your opponent gives, you take. Unless you see a good reason not to.” – Bobby Fischer
In games between masters, most players will resign if they are even one piece behind since they understand the value of extra material. They know their chances to beat another master whilst being a piece behind is almost zero.
However, if your opponent isn’t a master you should play on because there is a reasonable chance that they might still make a mistake which you can exploit
It is important to understand the relative value of the pieces, which I discuss here: The relative value of the pieces