Meaning of “back sb up” - Learner’s Dictionary

back sb up

phrasal verb with back uk us /bæk/ verb
SUPPORT

B2 to support or help someone:

My family backed me up in my fight for compensation.
SAY IS TRUE

to say that someone is telling the truth:

Honestly, that's exactly what happened - Claire'll back me up.

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