Ad hoc communication means communication which was not planned in advance. Examples include improvised meetings, chat messages, phone calls, 1-on-1 chats at a person's desk, etc.
Ad hoc communication is usually an indication of a person not having easy access to information they need in order to do their job.
As a team leader, you should consider ad hoc communication as a symptom of an insight management problem, and constantly ask yourself "Why is this information not natively accessible to this person?".