Simple Definition:

Conscience is the inner sense or feeling that tells us whether something we think or do is right or wrong. It serves as a moral compass [a guide that helps us make ethical decisions]. For example, if you feel guilty after lying, that is your conscience telling you that lying is wrong.

Very Simple Definition:

Conscience is the feeling inside you that tells you if something is right or wrong. For example, if you feel bad after doing something mean, that is your conscience.