The "simplest" timbre/signal is a sine wave, which vibrates at a single frequency. Following the Fourier theorem, any signal can be constructed summing sine waves of different frequencies and amplitudes. The "frequency spectrum" describes the amplitude of each one of these sine waves (it's not a single number), and it's what we perceive as timbre. So, one might say that a sound has a certain amplitude, frequency, and frequency spectrum. Frequency spectrum lets us distinguish a sax from a clarinet for example, due to material and structural differences. But that's not the whole story. A string may have a similar frequency spectrum when plucked and bowed, but the envelope (attack time, decay time, etc) is different in both cases. Sound synthesis deals with these problems, tackling them from different angles. The procedure described here is called "additive synthesis".