Simple Definition:

Behavioural science is the study of how humans and animals behave (act or do things) in different situations. It examines the factors that influence our actions and tries to understand why we behave the way we do. For example, behavioural scientists might study how people make decisions (choose what to do) or how animals respond to certain stimuli [things that make them react].

Very Simple Definition:

Behavioural science is the study of how people and animals behave and why they do what they do. It helps us understand how and why we act in different situations. For example, it helps us understand why we choose certain things or how animals react to things.