Refer to figure.

The Difference in Depth Modulation (DDM) is a principle used by the ILS to define a position in an airspace. An aircraft approaching a runway will be guided by the ILS receivers in the aircraft by comparing the depth of the modulations.

As seen in the figure, there are 2 different parts of the ILS, the localizer and the glideslope.
The localizer has 2 lobes of 90 Hz and 150 Hz. The ‘ideal’ path where both the lobes are at equal strength is called the centreline. Any deviation from the centre line will result in a difference of strength received from the lobes. The further the aircraft moves away from the centreline, the bigger the difference in depth of the 2 lobes.

The glideslope has also 2 lobes of 90 Hz and 150 Hz vertically orientated. The ‘ideal’ path is where both the lobes are at equal strength and is called the ideal glideslope. Any deviation from the ideal 3° glideslope will result in a difference of strength received from the lobes. The further the aircraft moves away from the ideal 3° glideslope the bigger the difference in depth of the 2 lobes.