To Join Together, To Unify, To Match
transitive verb, ichidan verb
You know that the alternative kanji meaning for 合 is join, which is what we're using here. The verb form of join is to join together, and it can also mean to unify and to match.
あわせる
The reading comes from 合う, so if you know that reading you'll know this reading as well.
彼は相手のペースに合わせるのが上手いよね。
He's good at matching the pace of others, isn't he?
このセーターに合わせる上着がない。
I don't have a jacket that goes with this sweater.
ムリして人に合わせる必要はないよ。
You don't need to force yourself to fit in with others.