In the US, settlement is T+2, so if you are buying, the broker doesn't pay the cash/receive the shares until two days later, but they won't let you buy without

In the US, settlement is T+2, so if you are buying, the broker doesn't pay the cash/receive the shares until two days later, but they won't let you buy without cash in your account first, so cash for unsettled trades is a bunch of float.