A skateboarder getting ready to perform on a half-tube rink where there are trees in the background.
A skateboarder stands on top of a halfpipe about to jump down.
"About to jump down," is a rephrasing of, "getting ready to perform."
On a half-tube rink means on top of a half pipe and getting ready to perform means about to jump down.
A half-tube is a halfpipe.