The fastestmirror plugin downloads from the fastest mirror it can find, so it speeds up downloads. The leaves plugins show and/or delete new leaves when you uninstall a package. Say package C needs packages A and B, but no other packages on your system need A or B. You can safely erase A and B after you erase C. Type "yum info yum-plugin-*" without the quotes for a list of available plugins. You install them with yum, just like any other package.
The flash repo is where leigh123linux keeps the rpms for the current 64 bit flash plugin. If you are running 64 bit Fedora, see this thread
for more information.