EXP Boost lets you earn 1.5x experience when it is enabled, including EXP from quests and killing monsters. Click the EXP Boost button at the top-left of the mini map to open the window, where you can enable or disable EXP Boost and check the remaining time.
EXP Boost has a time limit, which will count down while EXP Boost is active. Each week, each player will receive 300 minutes of free 1.5x time. Remaining time will be lost, and the total reset back to 300 every Sunday. EXP Stones will grant additional Boost time, which will remain until it is used. EXP Stone time will only be used when the Weekly Boost time has been exhausted.