Resonance

Resonance is the natural vibrational frequency of a system. Stimulation at that frequency produces energy transfer without much loss. In the simplest cases, resonance frequencies are the frequencies that produces standing waves.
e.g., a swingset (pendulum) – kick your legs at a particular frequency to maximize the energy transfer; pipe organ; breaking a wine glass by singing
If you continue to add energy at the resonant frequency, the wave oscillations will grow quickly

Guitar strings play a certain note when you pluck them (1st harmonic or fundamental mixed with additional harmonics)
You can change that note by altering the length of the string (i.e., the wavelength of the first harmonic – which also changes the frequency)
You can also change the note by using a different thickness of string (i.e., the velocity of the wave in that medium will be different – and therefore the frequency)