Hint is a piece of advice that provides players an extra guidance to answer the task. If hint is present then icon is shown when task is open, if hint is not present then icon is not shown.
Usage of hints is counted in a game (usage is visible in online results page) and you can limit the amount of hints which players can use per game (to do it you need to go to “Configure” page while creating/editing the game and choose “Limit hints when available” under “Game settings”.