in first player games, if an eco round fails the players must forcebuy, meaning they need to buy what they can with the money than have.

If you force buy, then you're spending all of your money – though you didn't have much to begin with. You only force buy for 2 reasons:

1. You want to stop losing.
2. You want to make your enemy broke.

The word "Force buy" is mostly used in CS:GO and Valorant.