Focus

shortcut F Focus on Selection
shortcut CTRL + F in object mode isolate selection
shortcut CTRL + ALT + F in object mode isolate inverted selection

The Focus tools is used to either zero in on a selection of mesh parts or objects, or to isolate a selection of objects.

So it comes in two modes: View Selected and Local View. Both of these build on these native Blender tools, but improve them.

View Selected

In View Selected mode - called using the F key by default - Focus adds the ability to focus on everything, when there's nothing selected.
In object mode it will by default also ignore mirror modifiers, and so the view will only zoom in on the original, not the evaluated geometry.

Local View

In Local View mode Focus adds a history, allowing you to step up and down multiple levels of local views.
Whether you go up or down, depends on your selection.

By default, mirror modifiers will be disabled when stepping down, and re-enabled when stepping up.

Using Focus