Meaning of “assert” - Learner’s Dictionary

assert

verb uk us /əˈsɜːt/
assert yourself

to behave or speak in a strong, confident way:

She has to learn to assert herself.
assert your authority/control/independence, etc

to do something to show other people that you have power

[ T ] formal to say that something is certainly true:

[ + that ] He asserts that she stole money from him.

(Definition of “assert” from the Cambridge Learner’s Dictionary © Cambridge University Press)