All waves have certain properties. The three most important ones for audio work are shown here:
Wavelength: The distance between any point on a wave and the equivalent point on the next phase. Literally, the length of the wave.
Amplitude: The strength or power of a wave signal. The "height" of a wave when viewed as a graph.
Higher amplitudes are interpreted as a higher volume, hence the name "amplifier" for a device that increases amplitude.
Frequency: The number of times the wavelength occurs in one second. Measured in kilohertz (Khz), or cycles per second. The faster the sound source vibrates, the higher the frequency.
Higher frequencies are interpreted as a higher pitch. For example, when you sing in a high-pitched voice you are forcing your vocal chords to vibrate quickly.