The suspension is a way introducing a dissonant note on an accented part of the bar. The 4-3 suspension is one of the most commonly used. In this type of suspension, the third above the bass in a root position triad is delayed until the next beat by the introduction of the dissonant fourth above the bass.
In the example below, the third of the G major triad (b1) is preceded by the fourth above the bass (c2) which is dissonant with the fifth of the triad (if you are unfamiliar with giving notes a name and number - e.g. c2 - click on the basics button in the toolkit).
Suspensions follow the general guideline for resolution of tension that the dissonance should resolve downwards by step. It also introduces the dissonant note on an accented part of the bar so the suspensions must be introduced - or 'prepared' - by the suspended note appearing in the same voice in the previous chord.
In the example below, the c2 in the alto voice is prepared in the first chord, suspended in the second and resolved to b1 (downwards by step). The movement from the dissonant c2 to the consonant b1 is results in a pattern of tension-resolution.
The second half of this example helps show how the suspension derives from a chord progression with no dissonance. The tension of the suspended c2 is the result of delaying the alto voice in a chord progession from I to V. You will come across various other types of suspension in this website, particularly in examples taken from music by Arcangelo Corelli.