Sorry, I assumed you would know
Modularity is the ability to connect only the wires you need to the PSU.
A non-modular unit has this huge hydra of cables sticking out of it. If you don't end up using all of them (which you most likely won't), you have to tuck them away from your case's airflow and to keep things neat. It's not a huge deal but if your case is small, you may have space issues.
If your PSU is modular, then it's either fully modular and every cable can be disconnected from it at will, or semi-modular, where some cables are hard-wired and most of the rest can be connected and disconnected at your leisure.
Full modularity is kind of useless on my opinion since there are cables on a PSU which you will always, always need, e.g. the 24-pin motherboard connector, but a lot of people appreciate to have it.
The XFX 850W Black Edition is semi-modular. If you look at the pictures in the review I linked you to, you'll see that it has eight green connectors. If you keep scrolling down, you'll see the contents of the package, including the large collection of modular cables which the unit comes with.