When a rock falls into water, the water gets pushed and pulled around, creating little waves that spread out. This also happens in air. When something like a ukulele string moves, it squishes and stretches the air around it. The squishing and stretching of air then spreads around as a wave. When the air squishes and stretches quickly and the wave reach our ears just right, we call this sound.
The frequency of the sound describes how often the air squishes and stretches in one second, and its unit is called a hertz (Hz). So if you could hear a sound of 1 Hz, then your ears are noticing that the air is squished and stretched 1 time each second. If you hear 9 kHz (kilohertz), then your ears are noticing that the air is squished and stretched 9 thousand times per second. Some frequencies are called notes and we give them names like B, C flat, and F sharp.