Stammering affects the way people speak.
People who have a stammer often find it hard to get across what they want to say: they sometimes get stuck trying to say certain words, or are not able to say them at all.
They can repeat words or sounds without meaning to.
Having a stammer is something you have no control over, and can make you feel embarrassed or frustrated.
Stammering varies from person to person, and sometimes people who have a stammer can speak clearly for long periods of time without stammering.
It can also be called "stuttering", or having a speech impediment.
Stammering is more common in boys than girls, and around one in 20 children under the age of 12 in the UK will have a stammer at some point.
Currently there are around 150,000 children and young people living in the UK who have a persistent stammer, which means it won't go away.
There are lots of different speech and language therapists who can help people with stammers and teach them techniques to speak more confidently.
Often people who get help from doctors and speech therapists when they are younger, say it has a massive improvement on their stammering as they grow up.