A perfect fourth is an interval of five semitones.
For example, the interval between C and F is a perfect fourth as it contains five 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 fourth note on the scale is equal to five 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 "fourth" if it occurs between the first note and the fourth note on the scale (or more generally between two notes with two notes in between).