It's just a spring training homer, sure, but it's a Grady Sizemore spring training homer. We haven't seen one of those since 2010, or a homer of any kind from Sizemore since 2011.
Sizemore takes Rays' lefty Cesar Ramos deep to right field, and does so during a stretch where the Red Sox are essentially stress-testing his body to see if he's fit for the every day center field job. Sizemore has been playing almost constantly of late, with the Sox using him in official Grapefruit League games on March 15, 17, 20, 23, and in this game featuring the homer, on Tuesday the 25th. He's also participated in a minor-league game on the 19th, in order to keep him playing and simulate something akin to a regular season schedule.
It's just one homer, and it's still just spring training competition, but seeing him launch one as he gets into the stress-test of the spring is comforting. He still has a long way to go to prove he's healthy and not just healthy enough to be the Opening Day center fielder, but one thing at a time.