Accrual accounting is a financial accounting method that allows a company to record revenue before receiving payment for goods or services sold and record expenses as they are incurred.
The other form of accounting is "cash basis accounting". It records revenue when the goods or services are paid for. It means revenue is registered when the payment is actually received.
Small firms prefer "cash basis accounting" since it's simplest to implement. Accrual accounting is a bit more complicated and it might require help from accounting experts from time to time.