A woot shows that a member of the community likes the song that is playing currently! When you woot something, your avatar begins to dance.
A grab shows that someone likes the song or video so much that they have added it to their own playlist so they can listen to it again and share it with others.
A meh means that someone dislikes the track which is playing.
Below each of the buttons you can see the number of how many people in the community have performed each action and will give you a good sense for how the crowd feels about each song played.