An imbalance occurs when you have too much of some things and too little of others. If you put so much pepper in your soup that you can't taste the other spices, then you caused an imbalance in your flavoring.
It’s easy to remember the meaning of imbalance when you break the word into parts. You might recognize the prefix im- as a way to say “not.” Combine that with the familiar word balance, and you come up with something that is decidedly out of whack, or not balanced. It means that something is out of proportion, such as a trade imbalance between nations, meaning the amount of goods they sell to one another is not equal.