A bias is a built-in error that changes all the values of a measurement by the same amount or makes certain outcomes of an experiment more likely.

For example, either Sam is really bad at playing darts, or there's a magnet inside the dartboard in the picture that attracts all the darts to the top of the outer ring. A magnet in the dartboard would introduce bias.

For example, if you always weigh yourself while you're wearing a thick coat, you introduce bias into your weight measurements.