Plugins are tiny little "things" in Affinity, otherwise known as widgets, gadgets, applets, etc. They are displayed within Affinity and some are very integrated with Affinity. For example, the favourites plugin adds a new application to its list whenever you launch one from Affinity's search. Plugins use a variety of utilities found in libaff, distributed with Affinity. This allows them to use the buttons and frames that Affinity uses. Plugins can also directly access all of Affinity's settings, which can help a plugin fit into Affinity.
A list of plugins can be found on pages tagged 'plugin'.
Plugins can be enabled in affinity-preferences. There are five different areas into which plugins can go, and each area has two lists, one for the plugins at the top/left, and another for the plugins at the bottom/right.