A perfect fifth is an interval of seven semitones.
The interval between C and G, for example, is a perfect fifth as it contains seven semitones.
In most common scales, such as the major scale and the natural minor scale, the interval between the root of the scale and the fifth note on the scale is equal to seven semitones. This interval is called "perfect" as it is neither major nor minor (it occurs in both) and as it occurs in the most common scales. This interval is called a "fifth" if it occurs between the first note and the fifth note on the scale (or more generally between two notes with three notes in between).