What it says. The only merch we have of her shows a single jagged lightning bolt; this is, however, on her flight suit based on the paint job.

That said, it's not the standard Wonderbolts insignia, and her b-canon IDW comic appearances have her dressed in a lightning motif, giving credence to it being her actual mark.

Counterpoint: she's not the only mini fig in the Cloudsdale set to have a lightning flight suit (rather than the traditional Wonderbolt "winged lighting"). In fact, a slight majority of the suited ponies DON'T HAVE the official insignia and have the single bolt instead.

Yeah, Rainbow Dash has the single lightning bolt too, which might mean it signifies rank more than anything?

Just another detail for the fans to figure out, like Wallflower's cutie mark. "High Winds" does make me think it'll be something close to Windy Whistles'.

Spitfire merch has the single bolt, though, frustratingly. And she's canonically the highest honcho.