Accruals mean to record goods and services received or provided when they happen, regardless of when they're paid. That's it.

When cash is received in advance before the goods or service are delivered or preformed to the customer then it’s not earned until it is. Such money can be called prepaid revenue, unearned revenue or deferred revenue.

Whether something is an asset or liability depends on specific criteria that are met. When something no longer meets the definition of an asset or liability, then it's a revenue or expense, whatever the case may be.

When we don’t know the actual figure we must estimate it. Any difference between the actual and the estimate will either be under or over. The difference is recorded on the month the actual becomes known. There is no problem with this because the under or over estimate will usually not be significant or material.