Favourites are a one-click endorsement of an idea and also a personal bookmarking tool. They are a quick way for you to express your approval or support of an idea, without necessarily commenting on the idea too.
You can favourite an idea directly in the feed by clicking the favourite icon:
Once clicked, the favourite icon goes pink:
This also creates a story in the activity feed below the idea:
You can unfavourite an idea at any point by re-clicking the favourite icon.