The wikipedia page which he linked in his answer describes false tones as follows:
"The fundamental is chromatically discontinuous with the lowest 2nd harmonic reachable on a three-valve instrument ... [False tones] have a pitch between the normal range and the fundamental."
So for example, a three-valve brass instrument in BBb (i.e. BBb Tuba) would have the note Bb0 as its highest fundamental (or pedal) note. Its lowest playable first harmonic note would be E1 (with all valves pressed). Then the notes B0, C1, Db1, D1, and Eb1 are between the instrument's lowest and highest notes, but are not really within its normal playable range. These are referred to as false tones when played.
Adding a fourth valve which lowers the pitch by 5 semitones will add these five notes to the instrument's normal playable range, by extending the playable second harmonics down to B0.
As for the physics of playing these notes, I'll let someone more knowledgeable answer.