If she is a touchy type of person and has a good relationship with you, she may touch your arm constantly simply out of habit.
If you have noticed that she tends to do this with other people in her friend circle, there is a good chance that she tends to touch your arm simply out of habit.
However, if you notice that you are the only one that she tends to touch in this way, she may actually be showing signs of romantic interest in you.
Touch is often a way for a person to feel connected to someone else. It bridges the gap between them. It helps to establish rapport.
It is also often a way for a person to gauge whether the other person is romantically interested in them depending on how that person responds to their touch.