Unity uses the term handle to refer to a group of gadgets that can be interacted with for different purposes such as transforming an object.
The plugin however uses the term gizmo instead and reserves the term handle to identify a component that makes up a gizmo. So a gizmo is made up of a collection of handles.
A handle can be hovered by the mouse cursor, and if the left mouse button is pressed while a handle is hovered, you can drag the mouse to interact with the gizmo. Basicly, handles are the means by which the user can interact with a gizmo.