A boom is a very loud, deep sound that echoes. The distant boom of fireworks might spook your nervous dog.
Guns, thunder, drums, deep voices — all of these are capable of a pretty loud boom. When they make the sound, you can also say that they boom: "I saw the flash of lightning and got ready for the thunder to boom." Another kind of boom is a prosperous period, like an oil boom or an economic boom. The long pole that holds a microphone or the sail on a boat is yet another type of boom.