The graphic below represents an aeroplane flying what is called an "instrument landing system" (ILS) approach.

Note how the electronic beam from the runway fans out laterally. It does this vertically as well, it is just not as obvious. The pilot is given an instrument to fly this electronic localiser (lateral guidance) and glideslope (vertical guidance) down to the runway. Typically the glideslope angle is three degrees in the vertical frame and 1.5 degrees laterally.
It becomes very sensitive when flying close to the runway.

The ILS is an aid to allow pilots to land at airports when the visibility is poor or there is low cloud nearby.

This system when approved by the airline is required for automatic landings.

On the Primary Flight Display (PFD), shown on the right, there are two magenta coloured diamonds. There is a lateral scale (called localiser) shown at the bottom, and a vertical scale (called glideslope) next to the altitude display.

The diamonds indicate where the plane is in relation to the ILS beam. It is a command instrument, showing here the plane to be above the glideslope and slightly to the right of the centreline (localiser). To be exactly on the correct ILS beam the diamonds should be in the center of their respective scales.
So the pilot here would descend a little quicker and fly slightly to the left to regain the correct course.