What is the name of the amount of money the brokers keep?

How do you calculate it ? for example, if i get like $5 profit, then i have to minus a certain amount of money, what’s the name of the amount that the brokers hold and how to you calculate it. I understand the value increases ( the amt they hold increases) as you increase your volume (btw i assume volume is the amount of money your’re willing to win/lose per move).

Brokers hold margin it’s a kind of collateral that enables you to open position which notional amount is substantially higher than the money you have. The idea is to guarantee that you are able to pay for the current loss of your position and this loss at any time can’t exceed your margin.