Welcome to PF ashyein!
I'm no expert, but the first place where I would start is to look at the derivation for (and properties of) the wave equation, especially in the context of transverse waves traveling on a string. Then, you can go from that to understanding standing waves on a string of length L with fixed ends, and in particular, why only certain frequencies (that depend on L) are allowed on them. The lowest frequency is called the fundamental frequency, and the higher ones (harmonics) are all integer multiples of it. The measurable physical quantity, frequency, is related to the human perception of pitch. Harmonics are equivalent to what musicians refer to as "overtones." That's all I can tell you.