A company has transactions every day throughout the year. When the end of a period comes the company should "close the books."
To prepare the financial statements at the end of an accounting period, a number of actions should be taken which represent closing the books. Closing the books process can be illustrated in the following way
http://simplestudies.com/what-accounts-are-not-closed.html

Closing the Books
At the end of a fiscal year once all the transactions for the entity have been recorded,
the revenue and expense accounts must be closed out to a zero balance. These
accounts have been accumulating balances over the period and they need to be set
back to zero so that the next period’s amounts can be accumulated.
http://www.dermottcrofton.com/uploads/2/8/2/8/2828730/7a_-_c...
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Each monthly accounting period goes from the first to the last day of that month. ... of period 14 is to accrue all year-end entries necessary for closing the books ...
http://www.pdx.edu/financial-services/book/export/html/204

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About closing your books at year-end. You close your books by setting a closing date. You can also limit access to the closed accounting period by setting a
http://support.quickbooks.intuit.com/support/pages/inproduct...

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The process of closing the temporary accounts is often referred to as closing the books . Accountants may perform the closing process monthly or annually.
https://www.boundless.com/accounting/textbooks/boundless-acc...