Do you know this person well? It can be used as a flirting technique - meant to be natural response to something funny or touching..... reaching over and touching a forearm. I'm not sure about stroking the arm though.
Is it a friendly gesture? If you did it to someone would you consider them a friend/care about them?
From a woman to a man, it can be viewed as a friendly gesture with no strings attached. However, from a man to a woman, it can be viewed as sexual assault from a deranged predator, who needs to be jailed for being a pervert.