No forward chain?

We're just browsing houses for sale in the UK and keep seeing "no forward chain."

What does that mean?

the person you're buying the house from doesn't have to wait to move (i.e., they are buying an empty house -- no upper chain)

If you can try to avoid getting involved in a chain. MAJOR HASSLE!!

I know this has pretty much been answered but being an estate agent, I thought I should add my thoughts...
Being in a chain isn't completely evil but always advisable to try and avoid if at all possible.
Say for example you are buying a house, and the seller of that house is also buying, and then their buyer is selling; If the person at the 'top' of the chain pulls out of the sale, the whole thing can go pear-shaped, unless of course the estate agents at the top can quickly find a replacement buyer. Chains aren't necessarily bad, they just have a higher fallout rate.

I've been in the business six years, so if you need any more advice let me know!  .