This is a very common disorder, and a frequent cause of dermatological consultation. It is these "bumps" which usually appear in the posterior area of both arms, that in summer tend to "light up" more and sometimes leave a residual post-inflammatory hyperpigmentation.
In more severe cases, patients may get the same lesions in the thigh or gluteal area.
It is merely a skin hydration problem, and this should be emphasized in its treatment. There are also specific topical treatments to help aesthetically with such lesions.