Are your investments liquid or illiquid? When a holding is liquid, you can sell it for a fair price whenever the market in which it trades is open for business. If it’s illiquid, you can’t.
Cash and cash equivalents (such as bank accounts and T-Bills) are the most liquid assets of all. At any time, you can typically find somebody who will gladly give you something worthwhile in exchange for your cash.
At the other end of the spectrum, some investments are highly illiquid, which means your ability to trade in and out of them whenever you please is limited.