Zoom In will increase the magnification of the model in the active view, so that the new view area is 90% of the old view area.
Zoom In is useful when you quickly want to zoom in on the model. You can keep selecting Zoom In as many times as necessary. If you want to zoom in on a specific area of interest, then use the Zoom Window option, rather than Zoom In.
Zoom In is available in:
The Zoom sub-menu of the View menu
The F5 function key
The Home key
If you have a mouse with a wheel you can zoom in by moving the wheel forward.