Prompting is important when we try to teach a skill and get an individual using AAC to use their system for a specific purpose. There are different levels or intensity of prompts and we use different types depending on the individual and whether or not they are familiar with the skill being addressed. Technically:

Prompting is the act of trying to make someone do something. A prompting hierarchy is a systematic way of providing prompts to help an individual learn new skills or practice a skill they already know.