The phrase "to clear the board" means to eliminate or remove all obstacles, problems, or challenges so that a situation can start fresh or begin again. Full definition
Clear the board in less than the given amount and you earn points towards earning stars with three stars being the most you can earn for each level. (thatvideogameblog.com)
To progress through the levels you have to clear the board by using all of the letters at least once. (ipadapps4school.com)
Each level is more challenging than the last, while the new power ups help clear the board, remove similar tiles, or blow up tiles in a given area. (crackberry.com)