Full Buy:

A full buy is a round in which a team spends enough money to equip most or all players with the best available weapons, armor, and utility. In Counter-Strike 2, it usually happens when the team wants to play a high-commitment round with a complete loadout instead of saving money for a later buy.

Full buys matter because they give a team the strongest chance to win a round, especially on important gun rounds after an economy reset or at the start of a half. Teams often call a full buy together so everyone enters the round with similar firepower and utility.