Maple leaf bladder gall
The gall is yellow to red when it starts out, then turns brown as the year wears on. The cause is a mite feeding on the leaf. The leaf then swells in response to the injury and toxins put into the leaf. The "bumps" are actually leaf tissue. I usually compare it to you getting a mosquito bite. There is no treatment, since the swellings are actually leaf tissue, and there is no damage other than the unsightliness.