Stuttering has visible and invisible forms. Blocking on letters and words and repeating of same are for others hearable. Making grimaces while speaking is visible for the person you are talking to. These are for the outside world typical forms of stuttering.
Speaking in front of a group is not made for everybody. There are people that love standing on a stage and getting the attention from a group of people. For people who stutter, this can be a nightmare.
Stuttering has a physical cause with mental consequences. It usually starts during the 2ndand 6thyear. Toddlers at this age have periods in which they speak less fluent: about 4% repeats words or letters and stumbles over what they have to say. Most kids grow out of this but 1% stays with a stutter problem.