Intrapersonal communication is communication that a person has with him or herself.
Although some might take the position that no communication is needed if only one party is involved, we communicate with ourselves all the time.
Intrapersonal communication can encompass:
Sense-making (see Karl Weick) e.g. interpreting maps, texts, signs, and symbols
Interpreting non-verbal communication (see Albert Mehrabian) e.g. gestures, eye contact
Communication between body parts; e.g. "My stomach is telling me it's time for lunch."
Speaking aloud ("talking to oneself"), reading aloud, repeating what one hears; the additional activities of speaking and hearing (in the third case of hearing again) what one thinks, reads or hears may increase concentration and retention.
Writing (by hand, or with a wordprocessor, etc.) one's thoughts or observations: the additional activities, on top of thinking, of writing and reading back may again increase self-understanding ("How do I know what I mean until I see what I say?")and concentration. It aids ordering one's thoughts; in addition it produces a record that can be used later again. Copying text to aid memorizing also falls in this category.
Making gestures while thinking: the additional activity, on top of thinking, of body motions, may again increase concentration, assist in problem solving, and assist memory.