Mostly a concert with a "name" country star or a somewhat has-been rock star. It's included with the price of admission, like the Mardi Gras shows at Universal or Flower Power shows at EPCOT. Unlike those shows, Sea World starts sometime in the late afternoon--I'd have to check but I want to say 4:00, maybe 3:00.
Concerts are at the lakefront stadium. If you go, get there early. Last year, Sugarland filled up the stadium at least an hour before showtime, although SW also set up giant TVs with the show along the path that runs beside the lake. There are also booths along that same path selling bar-b-que and the standard Bud beers. Same bar-b-que they normally sell in the park; it's just ok, nothing I'd go out of my way for.
I think the first artist is Trisha Yearwood, may be this weekend. Honestly SW has been all about Aquatica lately, have not heard a single ad for Bud n Bar-B-Que, which is unusual.