Having a means of functional communication is extremely important. This is the way the individual expresses and responds to their environment, needs, wants, desires, interests, concerns, pain, etc. Often, not having a way to express oneself is a cause of undesired behavior. For example, if the individual does not know how to say "I have a headache", and is being asked to complete a math sheet, she might scream as a way to get out of (or escape) doing the math sheet. She does not have a way to tell the teacher she does not feel well. It is likely this behavior has allowed her to get out of doing other things in the past and meets her need in that situation. This behavior has become her way of (non-functionally) communicating her discomfort. Other points to note:
Meaning matches what is being expressed. For example, if the individual verbally asks for an apple, they want an apple and not a hamburger.
Can be in any form of communication including verbal, picture exchange, written, sign, or through the use of technology.
Lack of having functional communication is the cause of many undesired behaviors.