Stands for Modular Cables.
If you have a fully modular PSU, ALL the cables can be plugged in and out the PSU. Semi modualr only the ATX 24-pin cable is attached , all the other cables are loose. Non modular PSU has all the cables to the PSU.
Its not important in terms of reliability or performance, only if you have a small case with bad cable managment it can be handy.