expression, godan verb (-su)
let someone go first; let someone pass ahead
Used when allowing another person to go through a door, walk ahead, or proceed before oneself. Often a polite gesture.
お先にどうぞ。先に通します。
Please go ahead. I'll let you pass first.
彼は私を先に通してくれた。
He let me go first.
先に行く means 'to go first' (oneself), while 先に通す means 'to let someone else go first'.
Compound of 先に (saki ni, 'ahead, first') and 通す (tōsu, 'to let pass').