A more thorough explanation:

A business day is a day on which most institutions are open for business, usually a day on which banks and major stock exchanges are open, excluding Saturdays and Sundays.

For example, if a company needs to transfer funds from one account to another, they need to do it on a business day. If they try to do it on a weekend or holiday, the transfer won't go through until the next business day.

Another example is the stock market. The stock market is only open on business days. If someone wants to buy or sell stocks, they need to do it on a business day.