Here's some information about the Masters Tour Qualifier system in Hearthstone.

The Masters Tour is Hearthstone's version of the Pro Tour/Mythic Championship. Beyond the Masters Tour, there's also the Grandmasters (Hearthstone'MPL equivalent) and the world championship.

Qualifiers for a given Masters Tour stop will run for a couple months online. In order to participate, you'll need to create an account on Battlefy (Blizzard's partner who provides the platform for the events) and link it with your Blizzard account. There's also an official Discord server for the Masters Tour Qualifiers, where you can receive important announcements, get in touch with the tournament admins, and chat with your fellow competitors.

Registration for each qualifier starts two weeks before the tournament. They are free to play. Register early if possible, since they can cap. Be sure to double check the region of the event you're registering for; while you can play in qualifiers for any region, your collection is not shared between regions. You'll also need to check in and submit your decklists before the event begins. When you're paired, you'll add your opponent to your friends list and play your match, reporting the outcome on Battlefy. If you win some number of matches, you will receive packs within 48 hours of the completion of the tournament based on your final standing. The overall winner of the event will be qualified for the associated Masters Tour. You can also qualify by achieving 5 qualifier top 8s over the course of the season or finishing in the top 16 unqualified on the ranked Standard ladder. After you qualify, you'll be responsible for your travel and accommodations, but every player gets at least $500 prize for showing up.