English speakers use the sound "uh" when they're not exactly sure what to say next. You can use it in many different locations in a sentence:

Uh, yeah, I think that's right.

You, uh, soak the beans overnight, and then you cook them for, uh, for about two hours.

It was, uh, uh, last December, I think.

People don't usually say "uh" on purpose. It's a habit which can be annoying if someone over-uses it. If someone wants to sound very intelligent, they may try to train themselves not to use "uh" as much.