A wave start releases runners in separate groups at staggered intervals rather than all at once. It's used in large races to ease congestion at the start line and along the early part of the course.
Runners are divided into groups, or waves, and each wave starts a few minutes after the previous one. It spreads the field out instead of releasing everyone at once.
Usually by predicted finish time, bib category, or registration order. Faster or earlier-registered runners are typically placed in earlier waves.
No, because chip time starts when you personally cross the start line, not when your wave is released. Your result is unaffected by which wave you're in.
They reduce crowding at the start line and on narrow early sections of the course. This improves both safety and the overall runner experience.
Sometimes, at registration, based on your expected pace. Otherwise the organizer assigns it for you.