It is very important that players alert their own bids before making an alertable bid.  This explanation goes to the opponents only, and your partner not only doesn’t see it but also doesn’t know you have alerted.    If you forget you can click on your bid and you are given a chance to write the explanation there, or you can send a chat message to the opponents with an explanation.

Paul Gibson of Berwick Bridge Club has posted a number of videos including this one on Alerting.

Another excellent Andrew Robson video is available on two topics, undo and alerting, if you click here

Undo:  When you have clicked on an incorrect bid and realise your mistake immediately.

Alerting: BBO is unusual in that you alert your own bid!!!  Seemingly entirely wrong but only your opponents see the exlanation that you give..  Click on the number, then fill in the explanation and finally click the denomination for your bid.  2,  Stayman,  clubs.

From the BBO forum Guidelines for Alerts:
All members should understand that, due to the fact that we have members from all over the world as well as members of all levels of ability and experience, not all players will agree on which bids should be alerted and which bids should be considered "standard".

The management of BBO is not going to get involved with trying to make rules in this area. It is up to our members to try their best to provide their opponents with information that may be helpful to them. Always remember that it is against the laws and spirit of the game of bridge to conceal information about your partnership agreements from your opponents.

The BBO software is designed so that players alert their own bids. This is called "self-alerting" and it is opposite to the approach that is used in live bridge clubs and tournaments. If you have any doubt as to whether one of your bids should be alerted or not, it is appropriate to alert.

If an opponent asks you for the meaning of one of your bids, you are expected to answer them politely, even if you think the answer is obvious. An appropriate answer can be "I have never discussed this with my partner". You do not have to tell the opponents how you intend your bid - only what you have agreed with your partner. It is innapropriate to use chat to explain your bids to your partner unless you get permission from the opponents first.