Sweep cash is typically a cash balance "swept" from your broker's non-interest paying cash base for your account to an interest-paying, usually FDIC-insured if in the US, account. The broker controls when money is moved from the brokerage account, or from the interest-paying account, based on their need to use the cash for your transactions and fees.
If you're in the US, and you have money in a sweep cash account, you may want to ensure that account is not with a bank where the size of the account, combined with other accounts you might have with that bank, exceeds FDIC limits. Often brokers will let you pick who the sweep cash account is kept with to avoid FDIC limitation violations.