Player absence happens when one or more players does not attend or participate in a role-playing game session that has been scheduled by their group. Depending on a group's social contract, they may play their session without the absent player(s) or cancel it (perhaps so they can rearrange to a time that is more convenient for all players in the group). In a game with a GM, a GM absence will almost always lead to a cancellation, although in very rare cases a replacement GM may be possible.
If a player knows that they will be absent for an upcoming game, it is their responsibility to let the rest of the group know as soon as possible. It is rude for a player to not inform their group that they will be absent for an upcoming game if they know about it in advance. Some absences have unexpected causes (e.g. health reasons, family emergencies, travel disruption), and can't be predicted, but it is still good practice for a player to let the group know what has happened when they get the chance to do so.
Repeated absences by the same player may indicate a lack of engagement with the game. Depending on the social contract, it may also be a lack of respect for their fellow players. However, some games (especially those designed for open table play) expect a certain level of player absence in each session.