Like: Agreement, acknowledgement, or otherwise nice thing that doesn't fit any of the other ratings.
Love: Too perfect IMO, major agreement, or I just like you a lot and where most people would get a like, you get this.
Thank: Thank you. Sometimes used when somebody answers a question really fast.
Useful: This was helpful to me and made my day/life better, but somehow doesn't warrant any of the other ratings.
Bucket of Rainbows: Generally used for LOL, amused, you're being a goofball, you're being a ray of sunshine, etc. Also used when I feel a little goofy or when love doesn't do the trick.
Go Home, You're Drunk: I've never used this rating, to be completely honest. The only times I've come close are when folks are talking about alcohol or being complete off-the-wall goofy people, though.
You Get A Cookie: It can mean anything, though I generally use it for a sarcastic "good job" or a "good to know" rating.
You Need A Hug: Well, you need a hug. I want to hug you and make you feel better. When I use this rating, I'm trying to show you love and nothing else. ALL MY HUGS ARE SINCERE MKAY
Nice Execution: Well done/well written post/awesome pun or whatever. Also used when somebody dies in a post because why the flop not.
These can vary heavily depending on context, but those are the general meanings I have for them when I use them.