Why does my computer eat my letters sometimes?

OK, I am typing along......I go back and read what I have written and I want to correct a word I mis-spelled or want to add to a sentence or add a word. A lot of times, when I insert something into a sentence, my computer becomes a "Pac-Man" and begins eating the letters to the right of what I have inserted. The more I try to correct the problem, the more letters the computer eats and I end up having to begin again.

Why does this happen?

Paulette Tweet