Crosstalk happens when there are multiple independent conversations happening simultaneously at a table, and can cause disruption to a role-playing game (especially games that are played online). Generally, a conversation among two or more players is considered crosstalk whenever the GM is engaged in a separate conversation about the game with another player, regardless of whether the conversation is relevant to the game or not (i.e. table chatter).
Crosstalk is considered disruptive, disrespectful, and can be particularly unwelcoming for people with hearing loss or other difficulties processing sounds and communication. If some communication is needed, it's recommended that this is done silently, e.g. by using a messaging app.