A generally positive exclamation, along the lines of "way to go" or the slightly older "excellent!." Originally from the Jonathan Coulton song Flickr and popularized by the podcast Bad Philosophy. Also can be used as an agreement.

Someone who posts a comment on Youtube Videos saying something stupid or obvious.
However, some retarded people really thumbs up these comments.

the act of emulating the motion of "jacking off" without actually jacking off while having the thumbs in the erect position

A person on Youtube (or likeminded websites) who submits a comment for the sole intention of getting "thumbs up".

Many of these people copy other thumbs up comments to get their attention fix.

An action used to express mutual excitement, where two people exchange props with their thumbs up.

A person that makes it a sport to get a comment into the highest rated comment section on youtube by posting a message that adds nothing to the video but is guaranteed to receive many thumbs up. Examples include, but are not limited to:
1) abusing an old and tired internet meme
2) using phrases that begin with "Thumbs up if..."
3) if the video was bad, pointing out that watching the video was a waste of time
4) squeezing Justin Bieber into the formula

Two Thumbs Up was born out of a need to express and extreme delight in a situation / person / movie / music / etc etc.

If something is good it gets a thumbs up.

if something is indescribably brilliant - it gets a 'Two Thumbs Up'

But it's simply not good enough to just say the phrase - you must also physically represent the Two Thumbs Up in a strong motion with both hands.

To be used sparingly.