A “bye” is simply a request for a player to miss a round without being withdrawn from the tournament.
In person events
For most local tournaments, one 1/2 pt. bye is available for any round that you must miss, if requested before the start of round 2. Additional byes will be at 0- pts. Some multiple-day tournaments will allow additional 1/2 point byes. Requesting byes is not the norm. Most players will play all rounds in an event and request no byes.
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Online events
Many online events, including tournaments held on ChessKid.com, do not allow byes.  Please be prepared to play all games.