A near menu is a collection of buttons on a panel that follow you with tag-along behavior. It stays in a certain range of distance and angle to allow you to interact with content. You can use the pin button or grab and place the menu to make it world-locked and disable the tag-along behavior. This allows you to focus on the target content without being disturbed by the floating menu.
You can re-enable the tag-along by pressing the pin button and make the menu follow you again.
Near menu in MRTK (Mixed Reality Toolkit) for Unity
MRTK provides various near menu prefab examples.