The prism system turns what would otherwise be an upside-down image right-side up.

Roof Prism System: The prisms overlap closely, allowing the
objective lenses to line up directly with the eyepiece. The result is a slim, streamlined shape in which the lenses and prisms that magnify and correct the image are in a straight line.

Porro Prism System: The objective or front lens is offset from
the eyepiece. Porro prisms provide greater depth perception and
generally offer a wider field-of-view.