Account is a synonym for the usual term wallet and can be described as a private account owned by a Monero user.
An account contains all of the Monero transactions that have been sent and received. The account balance is a sum of all the XMR received, less the XMR sent. Every Monero account has two balances: a locked and an unlocked. The unlocked balance contains funds that can be spent immediately, and the locked balance contains funds that can not be spent at the moment. You may receive a transaction that has an unlock time set, or you may have sent some Monero and are waiting for the change to come back to your wallet, both situations that could lead to those funds being locked for a time.
A key difference between traditional electronic currency and Monero is that your account resides only under your control, normally on your computer, and cannot be accessed by anyone else if you practice good security.