Space
Space is the territory your pawns and pieces control; the side with more space can shift pieces more freely.
Advanced pawns push the opponent’s pieces backward. Knights that should stand on f6 or c6 get kicked to e8 or a6. The extra room is useful only if you can still defend the pawns that created it.
White’s pawns on c4, d5, and e4 take the center. Black is in a King’s Indian structure: less space, but a solid kingside and a later break with …e6 or …c6 to chip at d5.
If you grab space and then stop developing, the opponent breaks and your pawns become targets. Space is an advantage when the pieces behind the pawns have squares.