What do you call a word that a person always uses in when talking?

They're often called a "verbal pause." A verbal pause is when you add in words that don't add any meaning but whose function is mostly to give you time to think. If you actually stopped talking to think, the listener might think you were done, so such verbal pauses cue the listener that you've got more you want them to hear before they answer. We often use them unconsciously.

I don't know of a specific word or phrase that describes this, but i think of this as "filler speech".

It's probably because you want to say something, but aren't quite ready and are perhaps nervous.

Just try not saying anything until you have thought out exactly what you mean.

Annoyance

Stalling

They are space fillers so you can continue to speak without interruption. Uhmmm, right?, and, or so are some others