Look at the tittle. One characters name is in all capitals, while the other two dont get capitalized whats implied?

This implies that Master Harold is the main character of the story. Another interpretation could be that Master Harold is superior to "the boys," both middle-aged servants of African descent. 

this is all based on the racial barrier, you have an 18 yr old being called master by the mid-40 yr old black men (sam n willie malope). just because hally is white he is being callled master by the two african man "AT YOUR SERVICE MASTER HAROLD"-willie.