What does this mean?

[GreenMeerkat]

Seen it on lots of threads, mainly in AIBU or threads that are deliberately provocative.

Just wondering what it means

It's a biscuit. Many varieties of its meaning though:

Your post takes the biscuit
Shut up and have a biscuit
No comment (Gordon Brown would not be drawn on his favourite biccie)

Generally it means 'You are being a twonk and I disagree with your sentiment'