This is the 2523rd most frequent Japanese word.
合わせる
"合わせる" means "to match, combine, or adjust."
Here '合わせる' means to adjust or set to match.
時計の時間を合わせる。
Adjust the time of the clock.
Here '合わせる' is used to mean matching or synchronizing.
一緒に歌うときにテンポを合わせる。
Match the tempo when singing together.
In this sentence, '合わせる' refers to combining or joining forces.