Wireshark supports plugins for various purposes. Plugins can either be scripts written in Lua or code written in C or C++ and compiled to machine code.
Wireshark looks for plugins in both a personal plugin folder and a global plugin folder. Lua plugins are stored in the plugin folders; compiled plugins are stored in subfolders of the plugin folders, with the subfolder name being the Wireshark minor version number (X.Y). There is another hierarchical level for each Wireshark plugin type (libwireshark, libwiretap and codecs). So for example the location for a libwireshark plugin foo.so (foo.dll on Windows) would be PLUGINDIR/X.Y/epan (libwireshark used to be called libepan; the other folder names are codecs and wiretap).
On Unix-like systems:
To provide better support for binary plugins this folder changed in Wireshark 2.5. It is recommended to use the new folder but for lua scripts only you may continue to use $XDG_CONFIG_HOME/wireshark/plugins for backward-compatibility. This is useful to have older versions of Wireshark installed side-by-side. In case of duplicate file names between old and new the new folder wins.