The start view is how your map is framed the moment a visitor opens it: how far it's zoomed, and the point it centres on. Getting this right means people land on exactly the part of the map you want them to see first.
The default: fitted and centred
If you set nothing, your map opens fitted to the whole level and centred, so the entire image is visible at once. For many maps that's the best possible first impression, and you can leave it alone.
When to change it
Change the start view when the whole map fitted on screen is too much at once. A large campus or a busy floor often reads better opening zoomed into the main entrance or a key area, letting visitors zoom out to see the rest.
Where the controls live
The opening zoom and focus point are set on the Levels tab, under each level's Start view. There you'll find Start zoom and Set focus for desktop and for mobile. Setting it per level means each floor can open framed on its own, and it's the one place that decides the opening view. See Set a different opening zoom for each floor for the step-by-step.
Fixed maps
If you turn Enable zoom off on the Settings tab, the map is locked at its start view and can't be zoomed or panned. That's handy for a simple, poster-style map. See Turn zooming and panning on or off.