Float is the money in a bank that has usually been credited to the bank, but not the customer.
Let’s assume a bank as an incoming payment of $100,000. Instead of crediting it out to the customer, the bank can keep it for one extra day and earn interest on it.
Almost every country has a system of overnight lending. Depending on how long this money the bank can keep, the money is lent out to other entities and the interest earned on the money. This money is called float.
This is the most common definition of float.
A more formal definition is presented by Investopedia: Float