Realised P/L is the profit or loss that you have secured so far, during the span of your position.
P/L can be secured by (partially) closing your position, receiving a rebate, incurring a fee or paying and receiving funding. Realised P/L is final. It does not change as the price moves.
Unrealised P/L, on the other hand, is the profit or loss you would gain from closing your entire position at the current Mark Price. This changes continously as the mark price goes up and down.
As Unrealised P/L isn't final, it is not part of your margin balance and cannot be withdrawn.
Unrealised P/L becomes Realised P/L when you fully (or partially) close a position.
Realised P/L is part of your margin balance and can be withdrawn.
For a more in-depth explanation, check out this article.