Margin is the amount of capital required to open a trade. Brokers and clearing firms use margin to make sure that there is enough money in an account to cover potential losses.
Understanding margin is very important as it allows the trader to manage risk and decide the number of contracts/shares you will use to enter in trades.
Every brokerage has different margin requirements based on their own risk. For example, you can find Interactive Brokers' entire margin structure here.
If you are with a different broker you can find the margin requirements on their website.