Meaning of “assure” - Learner’s Dictionary

assure

verb [ T ] uk us /əˈʃɔːr/
PROMISE

B2 to tell someone that something is definitely true, especially so that they do not worry:

[ + (that) ] She assured them that she would be all right.
MAKE CERTAIN

to make something certain to happen:

This loan should assure the company's future.

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