The principle of accrual (also called matching) is fundamental to accounting. It requires that costs should be matched to the revenues they generate.
This means, for example, that cost of an asset should be depreciated over its useful life rather than the entire cost being charged against profits when it is purchased.
It also means that sales are recognised when they are made, rather than when cash is received - so sales and purchases on credit are usually included in profit and loss account in the year the sale was invoiced in, rather than when payment is received.
There are more elaborate methods for dealing with issues such as long term projects that are partially completed. See revenue recognition.