a colloquial/slang expression meaning 'in an instant', 'immediately', or 'in a second'.
彼は宿題を秒で終わらせて、ゲームを始めた。
He finished his homework in seconds and started playing games.
新しいアプリは起動が早く、秒で反応する。
The new app launches quickly and responds instantly.
その冗談で私は秒で笑ってしまった。
I burst out laughing at that joke in an instant.
メールに秒で返信したから、相手も驚いただろう。
I replied to the email in seconds, so the recipient was probably surprised.