What does Helion's Hustle ability do?

Hustle is a traversal tool and nothing else. It sits in Helion's Mobility slot, which is the exploration side of the kit, and it works out of combat. The same speed boost also turns up as Helion's acceleration ability, granting high-speed sprinting at the cost of stamina.

The stamina drain is the whole balance of it. You are not setting a new permanent run speed. You are spending a resource for bursts of it, so Hustle is best saved for open ground where the extra sprint speed actually buys you time, rather than held down from the moment you leave a settlement. It applies across the open world, which makes it the main thing you will use Helion for while nothing is trying to kill you.

What Hustle does not do

Nothing in Hustle carries into a fight. Helion's combat work is done elsewhere: the Solar Grace passive raises damage and boosts its other skills, landing basic attacks builds a charge that strengthens the next skill, an empowered attack can stun, Holy Storm pulls surrounding enemies into a spinning storm with a 150% break multiplier, and the Ultimate, Helios' Judgment, deals heavy damage to nearby targets and temporarily evolves Helion into Soleon. Hustle contributes to none of that. Treat it as the thing that gets you to the fight faster.

Helion or Lunara

Helion and Lunara have identical base stats across all categories, so abilities like Hustle are the kind of difference that actually matters when you make the first major decision in the game. Both are Light-type Aniimo. The full comparison is in the Starter Choice Guide linked from this page.

Not settled: Helion's exploration speed boost appears under two names, Hustle and acceleration, and both describe the same thing: high-speed sprinting paid for in stamina. Nothing yet separates them into two distinct skills.