In this stanza, the writer of the poem is wondering what would happen if he died. He wonders if he died, if a "Kindred spirit" who also likes to ponder the lives of the dead (as he does) would come along and wonder about his life. He says that "Haply some hoary-headed Swain may say, / 'Oft have we seen him at the peep of dawn/Brushing with hasty steps the dews away/To meet the sun upon the upland lawn.'" Here Gray is simply saying that maybe someone (hoary-headed Swain probably refers to a farm laborer) will come by his grave and think to himself, "Hey, this is that one guy that I always saw early in the morning, brushing the dew away from his clothes (because he'd been out all night contemplating life in the graveyard), getting ready to meet the day". Gray, in his contemplations in the graveyard, or his nighttime walks, probably often saw farm laborers going out to their fields at dawn, so he is just wondering if anyone would remember those early-morning meetings.
I also provided a link to a complete interpretation and summary of the entire poem-that should be very helpful.