What does...

[Tash28]

This mean? What the hell is it?! I've been struggling with this for days and see it cropping up often!

[SquirtedPerfumeUpNoseInBoots]

its a biscuit.
it means "no comment".

[Fenton]

It's an insult usually, often awarded to an OP who has just posted something gobsmackingly obnoxious or ignorant e.g:-

OP: Well they just hand out blue badges willy nilly these days.

Response:

[Ferguson]

REALLY? I always thought it was a reward - I like biscuits!

[FloraFox]

**

[Trills]

If you look below the box where you type in your post you'll see that it says

No comment [biscuit.]

[KateSMumsnet]

The "official" meaning is no comment, it stems from the infamous Gordon Bronw webchat IIRC.