Accrued income is an adjustment made to the revenue of the company based on the fact that you have completed work for a client, but haven't necessarily invoiced them for it yet. It is considered to be an accounting adjustment that moves revenue back to an earlier month when the amount is considered to be been earned.
Sometimes if you're feeling generous, you will do work for a client before invoicing them. In these cases because you are required to record the income in the period you have earned it, you can create an entry in to the accrued income category that will increase your revenue for the value of work you have completed. The main reason you'll do this is when the work you've completed for the client straddles your financial year, meaning you have to record some of the revenue in your accounts that you haven't yet raised an invoice for.
What's not included
Accrued income is not to be confused with Deferred Income which is when you invoice a client up-front but haven't yet done the work for them.