BGG allows users to "recommend" particular content by clicking on the green "Thumbs Up" icon. A variety of content (for example, GeekLists on each game entry page) are sorted by number of recommendations, by default.
After you give a "Thumbs Up", the thumb icon becomes gray. You can undo a "Thumbs Up" you have given by simply clicking on the gray "Thumbs Up" icon to toggle it back to green.
When one of the front page modules is sorted to list the "hot" content, it lists those that have garnered the most recommendations .
The particular users who have recommended a piece of content may be viewed by clicking on the link that says "...has been recommended by N users". Further, the particular pieces of content that a specific user has recommended may be viewed in their User Profile.
The number of thumbs-up you have received from other users can be found in the contributions section of your User Profile. When you have received 100 thumbs-up you get a golden thumb and 1000 thumbs-up (10 golden thumbs) gives you a yellow meeple. A golden meeple indicates 5000 thumbs-up.