Or what's its equivalent?
The whole sentence was: "Let me give you a round up below the cut"
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There are two separate elements here. The first (round-up), which simply means summary, is a relatively common and established (informal) usage.
But below the cut is an unusual turn of phrase that probably only started to appear with the Internet. Cut here is jargon/shorthand for cut-off [point] - the limit at which something is no longer applicable.
Basically, the writer expects that his text will be cut off (terminated prematurely) in whatever context the reader will initially be reading it.
In this specific context, it's Charles Stross's blog page. He wants to get his summary in before the point on that page where it says something like Click here to read the rest of this article.
Edit: After posting this, I realised that logically, one might expect before the cut in OP's context. Have a look at How to do a text cut/”read more” on tumblr, where it says...
Everything of this post after the “read more” line will be put under a cut.
I'm not sure it really makes much difference to the overall meaning, but presumably the cut in OP's case actually means the above text, which has been "cut & pasted" from another source.