A comm is a form of communication with one or several people.
Basics[]
A comm is short for "communication."
There are several ways to comm another person, directly or publicly.
In Cinder, it's revealed that not many people use a "direct communication chip" or "D-COMM" anymore and the ones that still existed were very slow. In Cress, it seems that usernames could be personally edited as Cinder had changed her's to mechanic instead of the default one that had been given. Prince Kai commed other Earthen officials directly on portscreens, which might have been the traditional way to comm a person.
In Cinder and Scarlet, it showed that a comm could be one of two things: a vidlink, with people seeing each other and talking. Prince Kai comms other Earthen officials and Levana in this way. It could also just be plain text, such people texting each other or a notification. Cinder receives notification-like comms from the quarantines informing her about Peony's condition and Scarlet receives a notification-like comm from the detectives in Toulouse.