Unconscious biases are social stereotypes (including gender stereotypes) about certain types of people that individuals form outside of their own conscious awareness.
Unconscious bias refers to a bias that we are unaware of, and which happens outside of our control. It often manifests in a mismatch between intention and behaviour. So, while we may have certain beliefs (e.g. everyone is equal) our unconscious bias might betray those beliefs. Most of us are biased because gender stereotypes are so consistently reinforced by society and the media that our subconscious has accepted them as fact.