Select Object: Selects clicked objects to make them active for editing.
Selection Region: Defines the shape of a click-drag-release selection region. It defaults to rectangular, but clicking on the small tick in the lower right corner of the button will expand it to allow you to select a circular region or a fence region outlined by your mouse selections.
Selection Filter: Dropdown menu that allows you to define which classes of objects are affected by your selections. Defaults to all, but can be set so that only mesh objects, or lights, or other specific objects are selected.
Select by Name: Opens a second dialogue with a list of all objects in your scene, allowing you to select them by name one at a time or in multiples.