In this sense, feller means a man — a guy, a fellow. It is how fellow sounds when the last bit is weakened, the way some American and older British speakers say it: a nice old feller, a big feller. It is informal, a little country or old-fashioned, and often warm. You write fellow in careful English; feller is the spoken shape. Though actually, there is another feller that means a person who cuts down trees, from fell, “to cut down,” plus -er. That is not this word. This one is just fellow in work clothes.