The meaning of "keep someone in line" is:
The definition of "keep someone in line" is:
(verb) keep someone from doing something bad
I have to keep my younger brother in line while my parents are on vacation.
I kept my son in line while he was a teenager, but now I can’t stop him from doing bad things anymore.
A: Are you going to be a strict parent?
B: Yeah, I’m planning on keeping my children in line, so they don’t do anything stupid.