Simple Definition:

Confirmation bias is the tendency to interpret or seek out information in a way that confirms our existing beliefs or opinions, while ignoring or discounting [disregarding or minimizing the importance of] evidence that contradicts them.

For example, if someone strongly believes that all dogs are friendly, they may only pay attention to stories or experiences that support this belief, while dismissing or ignoring any evidence of dogs behaving aggressively.

Very Simple Definition:

Confirmation bias is when we only pay attention to information that agrees with what we already believe, and ignore anything that disagrees with us.

For example, if we think that all cats are lazy, we might only notice the lazy behavior of cats and not pay attention to any active or energetic cats.