What does the option "Allow participants to join anytime" means?

This means you are allowing participants to join the meeting you scheduled anytime.

It helps in case, you will be late to start the meeting. Take note, meeting will end after 40 minutes if you still have not join in by then.

But do remember that each account can only host one Zoom event at any time.

So, if someone else has joined the other meeting you previously scheduled and allowed participants to join anytime. You will be asked to close one of the meeting when you try to start another meeting.