A plug-in is an add-on module that extends Maya’s capabilities. File translators are plug-ins you use to import and export various file formats. You can create or purchase specialty plug-ins to customize Maya for a specific job.
Some features that can be added through plug-ins are:
Force Maya to unload a plug-in being used in the current scene
If you try to unload a plug-in that is providing functionality in the current scene (for example, the scene contains a node type that is provided by the plug-in), Maya warns you and gives you the opportunity to force the removal of the plug-in.
If you select to force Maya to unload the plug-in, Maya converts any nodes in the scene provided by the plug-in to the “Unknown” node type. Even if you reload the plug-in, the nodes remain Unknown.