The guy Mac DeMarco sings about in “Brother,” premiering today at The Atlantic, has probably seen better days. But the song is about as warm and soothing as rock music gets. “Take it slowly, brother,” the adventurous Canadian mews. “Let it go, now, brother. Go home.” Languid guitars sort of wade in the mix, just barely above an ambling bass line and a comforting 4/4 beat. DeMarco is only 23, but here and elsewhere on his forthcoming album Salad Days—out April 1 on Captured Tracks—he seems wiser than his years suggest. He seems to get that the simplest message—“take it slowly,” etc.—is often the most profound.