Glossary
Force buy
In one line: Buying the best you can with the money you have, even if it is not enough for a full buy.
What it means
Forcing means spending everything now instead of saving: cheaper guns, light armor, few abilities. You do it when a round has special value (for example after losing the pistol round, or at match point) or when saving would not get you a proper buy next round anyway.
The risk is obvious: lose, and you are broke next round. That is why a force buy is decided together: five players with SMGs and light armor can win a round, two forcing and three saving almost never do.
An example
You lost the pistol round but the enemy got few kills. In round two you call a force: Spectres, Bulldogs where you can, light armor. Win it and you break their economy; lose it and you eco in round three.