A note becomes 'suspended' when we delay its resolution. To grasp the mechanics of Suspended Notes, we need to understand how music moves melodically.
Although music shares similarities with 'the spoken language,' music is a gravitational language. In music, melodic and harmonic motion follow the 'law of gravity,' melody and harmony move through centers of stability, known as 'tonal centers.'
The tonal centers are the notes or chords with the strongest gravity pull. Imagine our solar system, the tonal centers would be the Sun, Jupiter, and Saturn because they have more gravity influence than the other planets due to their massive size, and the Sun is the largest and most massive, which is why all the planets orbit around it.
The same is the case with musical notes; some notes have more gravity influence than others, and the notes with more gravity pull are the notes of the triad chord. A triad chord consists of three Notes, the Root (1), the Third (3), and the Fifth (5)
The Root is the strongest them of all, and it is like our Sun in which everything gravitates around it; when we are playing the Root, we are in complete stability. We produce melodic and harmonic motion when we move away from these centers; that's when gravity starts pulling down the notes back to these centers.
Let's say we are in the C Major Mode. The notes of the scale of C Major are:
C, D, E, F, G, A, B, (C)
Now, let's say we are playing the C Major Chord: The notes of the C Major chord are C, E, and G. That means that C, E, and G will be the tonal centers and the most dominant notes.
(C) D (E) F (G) A B (C)
Therefore, all the other notes (D, F, A, B) will gravitate toward them. These notes are known as 'Passing Notes' and they tend to resolve to the closest tonal center.
If we play D, the note D will lean back to (C) or up to (E). If we play F, it will tend to go back to (E); or up to F. If we play A, it will lean back to (G) or up to (C), and if we play B, it will lean up to (C) or back to (G).
A suspension happens when we stay on a passing note and delay its resolution. When we pause on a note that is not a note of the chord a 'suspension' is created, which we'll eventually need to resolve.
(Please watch the video to see music examples of this concept)