Uncollected Funds of Savings Accounts, Explained:

Checks and other instruments that are on deposit, in the savings account, which are not yet cleared and available for use are considered to be uncollected funds.

Actually, the Federal Reserve Bank makes payment on the check before the issuing bank. The Fed Float defines the period between the Reserve making payment before the maker bank does. The time a bank has to pay checks depends mainly on the location of the Fed's processing facility where the check was issued, in relation to the one where it is being deposited. A process that can take up to five business days. The periods are outlined and required by Regulation CC of the Federal Reserve

If the issuers account has an available balance which amounts to less than that of the written check, there are said to be non sufficient funds.