Q: What does it mean when the word SUS is used next to a chord?
A: What it means is that the chord is suspended from having a quality such as major or minor. If we look at a D major chord, which has the notes of D, F#, A and keep in mind that to create a chord we simply jump over every second note in a scale. This will give us the first third and fifth notes, which is referred to as a triad. Since the first third and fifth came from the D major scale. We have in turn created a D major chord. To create a minor, we lower the third degree down a half step. This would give us the notes of D, F, A, which are the notes of a D minor chord.
So, as you can see, one of the most important notes in a chord, aside from the root, is the chord's third degree. The third degree is the note, which determines the chords quality. If the third comes from the major scale, we have a major chord. If the third comes from the minor scale, (or if we lower the third of a major chord), we generate and minor chord.
If you suspend a chord, what you really do is replace the third degree, with one of its surrounding tones. These tones can replace the third altogether, (by using either the second degree, or the fourth degree of the scale). If we replace the third degree, with the second degree of the scale, this type of chord is called a suspended second. If we replace the third degree with the fourth degree of the scale, this type of chord is called a suspended fourth.
Another type of chord often confused with suspended types are the add chords. An add chord, such as add2 or add9, added on a degree of the scale alongside the already existing triad. In a D major chord, the first third and fifth steps would remain as a part of the chord, with the additional degree added to it. For example; D Major (add2) would have the notes of, D, F#, A, E. although the terms; add2 or add9, are often used interchangeably, on ninth chord should contain a seventh degree, otherwise the chord should be written as D add2.
In summary, I would like to just make mention that as you study music and music theory it is a very good idea to begin learning about the degrees (or steps) of scales. This information makes up the building blocks for all single string theory as well as harmony. The more familiar you become with seeing your scales as a series of numbers (or degrees), the easier it will be for you to learn about all of your intervals, chord construction theory, and the theory necessary in order to improvise. An excellent book for applying this knowledge to guitar and further developing your knowledge of music is Music Theory for Guitarists. This book outlines a great deal of information and almost half of the book is dedicated to music theory itself. I highly recommend it.