People who stutter are terrified to answer a telephone, speak up in a classroom, apply in person for a job, ask for a date, order in a restaurant, etc. Because stutterers are afraid to talk, they are thought to be less intelligent than the average person. Some famous people who were stutterers, but overcame it: Thomas Jefferson, Charles Darwin, Jack Paar, and Anthony Quinn, to name a few.
When a stutterer stutters, here's what NOT to do: Don't insert words, or finish phrases for the stutterer. Don't say, "Just think before you speak, and don't talk so fast." These seemingly helpful suggestions don't work.
What can help? There's a National Stuttering Project composed of members who form self-help groups in every state. I have seen their materials and newsletters, and recommend them highly.