The Grid's plugins provide custom/extra functionality for the component.
Overall, plugins in Ext JS don't need to extend another class, but the idea/purpose is to give extra functionality and behavior to existing components.
For the Grid panel, we have some plugins already implemented in the Ext JS library. The most commonly used are Ext.grid.plugin.CellEditing and Ext.grid.plugin.RowEditing.
These two extend Ext.grid.plugin.Editing, and their primary function is to provide the grid with the ability to make cells and rows editable.
Note
Ext.grid.plugin.Editing should never be used directly because it is the base class for Ext.grid.plugin.CellEditing and Ext.grid.plugin.RowEditing.
Also, to make the columns editable, it is recommended to set the editor property in the column configuration.