In business, liquidity is defined as the period of time it takes to turn assets into cash. It takes 20 minutes to turn the balance in the checking account into cash. You head on down to the bank and present a check. You get cash. But most businesses run on just more than the cash in the bank account.
When a customer agrees to owe you money for your product or services provided, this is referred to as a receivable. Typically the customer is granted a short period of time to pay the amount due. Most receivables are paid within 90 days.