At the beginning of each player's upkeep, that player chooses target player who controls more lands than they do and is their opponent. The first player may search their library for a basic land card, put that card onto the battlefield, then shuffle.
The targeted player controlling more lands than the current player is a part of the targeting requirement. A player can't be targeted by this ability unless it's true, and the ability doesn't resolve if it's no longer true at that time.