A buckle or mechanical power strap uses a ratchet system instead of traditional velcro at the top of the boot cuff. It gives you the most precise and powerful closure you can get—there's zero stretch, so every bit of energy from your leg transfers directly to the ski.
This type of strap is really designed for racers and aggressive expert skiers who need maximum responsiveness and don't mind a slightly more complex closure system. If you're carving at high speeds or running gates, that locked-in precision can make a real difference.
For most recreational and intermediate skiers, a standard or wide velcro strap is plenty. The mechanical strap adds cost and complexity that you won't fully benefit from unless you're pushing the boot to its limits. And whatever strap you have, make sure it's actually snug—leaving it loose negates its purpose entirely.