Center control
Center control is occupying or attacking the d4, d5, e4, and e5 squares so your pieces have more room than the opponent’s.
Pawns on d4 and e4 claim space and keep enemy pieces off those squares. Knights on f3 and c3 support the same idea. You do not have to occupy every central square; attacking them is enough.
This is the Scotch: White has played d4 against …e5. The e5 pawn is challenged, and if Black takes, White recaptures toward the center and opens lines for the bishops.
A side that gives up the center without pressure on it often ends up cramped. The space page is the longer-term version of the same fight.