Alerting and Announcing - when a bid is not 'natural' is is necessary in most cases to let the opposition know.
Two ways of doing this are Alerting and Announcing.

Alerting is done either by the partner of the bidder immediately using an 'Alert' Bidding Card or equivalent,or, if bids are being made verbally, by making a positive tap on the table. An opponent, on their next turn to bid, may ask the partner of the bidder, to give an explanation of the bid.

Announcing is done automatically by the partner of the bidder immediately the bid has been made, to inform the opposition of the meaning of the bid and should give a brief explanation of partner's bid. An opponent may still, on their next turn to bid, ask for further clariication.

There are predetermined bids that require an Alert or Announce.

It should be noted that no alert or announcement is required for bids over 3NT other than non-natural openings above 3NT.

This does not prevent the opposition from asking what the bid means.