In equal temperament, the ratio between two frequencies as a function of pitch is
F(n) = F2/F1 = 2^(n /12)
F2 is the frequency of the second pitch,
F1 is the frequency of the first (in hertz) and
n is the difference between the two pitches, in semitones (1/12 of an octave).
So to compute the frequency of one pitch, you need the frequency of another. In this case they use A4 as the reference pitch, which is (normally) tuned to 440Hz.
The difference in semitones is the difference in MIDI note number, since there is one note number per semitone. So
noteNumber - 69 is the difference in semitones between the pitch and A4 (midi note #69).