Right click the
Preferences Icon: Get plugins

these are the ones show in the MR index of plugins. There is a short description shown (on bottom) for the one highlighted. But: there is a clickable link (right side, just above the summary) to the MR thread dedicated to that PI, for more info an usage/config tips).
Those are not ALL the Plugins you can use with Calibre, just the ones submitted here that might have wide appeal. The rest (from other places) are installed (or removed) via
Preferences: Plugins: (buttons at the bottom)