Music is made up of waves. When a violin player bows a string, the string vibrates at a certain frequency and creates a sound wave, which travels through the air, hits your eardrum and causes it to vibrate. Your brain interprets the signals coming from your eardrum and "hears" a sound.
Likewise, everything else you can hear is because something is vibrating and creating sound waves. In a trumpet, it's a column of air. With an electric guitar, the vibrating strings send a signal through the amplifier, which causes a speaker cone to vibrate in the same manner as the original string. When you speak or sing, it's your vocal cords vibrating. All of these things generate sound waves.
The properties of these waves affect how they sound. The frequency of a wave refers to how many times per second the wave transitions from its highest point to its lowest point and back again. This is typically measured in hertz (Hz), or number of cycles per second. The frequency of a wave determines its pitch. High frequency waves have a high pitch, and low frequency waves have a low pitch. The average human can hear frequencies from 15 or 20 Hz to roughly 20,000 Hz (20 kHz).
The amplitude of a wave refers to half the distance between a wave's highest point and its lowest. The larger the amplitude of a wave, the louder its volume, which is typically measured in decibels (dB). The decibel range for human hearing is complicated and depends on the frequency of the sound in question, but roughly ranges from 0 to 120 dB, with each change in 10 dB corresponding to a doubling of the perceived volume. (Yes, I know that 3 dB doubles the energy, but that's not the same thing.)