The term "pending" on a bank account means that the transaction is not fully processed through the Automated Clearing House. The ACH is a network of financial institutions that handles electronic monetary transfers.
Each time an electronic deposit is made, a paper check is presented for redemption, a debit or credit card is swiped or an automatic payment is made, the issuing bank verifies the account through the ACH.
Due to the enormous number of daily transactions and communication speeds, that ACH verification process can take one minute or take a couple of days. During the verification period, the term "pending" often appears alongside the transaction.
Once verified, the amount of debit or credit to the bank account is made and the term "pending" is vacated.