Bias of an estimator

Bias of an estimator is when something is not giving you an accurate answer. It means the answer is not exactly right, but it might be a little bit off. For example, if you ask someone how tall they are and they tell you they are 6 feet tall, but they are actually 5 and a half feet tall, that would be a bias in their estimator — they estimated (guessed) their height, but it wasn't exactly right.