Stuttering is a communication disorder in the rhythm of speech, where the person knows what they want to say, but in the moment is unable to say it due to involuntary:
Repetitions of sounds or words: m-m-my, where-where-where
Prolongations of sounds: ssssssometime
Cessation of sound: airflow is stopped, may be accompanied by muscle tension in the face and body
*All three of these characteristics may be present at any given time in the speech of a person who stutters.