When a tilt shift lens has no tilt, then it works exactly the same as a regular lens. I.e. the plane of sharpest focus is a vertical line at a distance determined by the focus distance on the lens. Increasing depth of field allows subject matter in front and behind the plane of sharpest focus to become sharper.

In very simple terms; when you tilt a Tilt-Shift or Perceptive Control lens, the plane of sharpest focus becomes a diagonal line that runs fore-aft. Increasing depth of field allows subject matter in above and below the plane of sharpest focus to become sharper. The goal is to tilt the lens so that the plane of sharpest focus goes through your foreground and background and then have enough DOF so that everything above and below that line is also in focus.

If you are using a wide angle (24mm) TS or PC lens then it can be very difficult to see all this in the viewfinder. When I’m using my Nikon Perceptive Control lenses then I use the live view mode to focus them which makes the whole process much easier.

I hope that helps!