During Brewfest, you can get a buff that increases your experience gained by 10 %. The buff lasts for 2 hours, so it's a pretty nice bonus when you are leveling up in The Broken Isles, or leveling an alt. It's only possible to get the buff twice per day, and in order to get it you must witness the Tapping of the Keg ceremony. This ceremony takes places outside of Ironforge and Orgrimmar at exactly 6.15AM and 6.15PM every day during Brewfest. This is server time, so make sure you don't get there at your local time.
Everyone that is close to the ceremony gets the buff, and there's nothing else to it than to be there at the right time.