Many people confuse soundproofing a room with another aspect of studio recording, mainly because the term for this other aspect is far more obscure, even though we see it all around us all of the time.
This is 'acoustic treatment' and there's a critical difference between treating your acoustic environment, and soundproofing a studio. Let's understand that difference between soundproofing vs acoustic treatment:
Soundproofing
Soundproofing is the act of drastically reducing or entirely blocking out noises coming from outside of your room. The key difference to note here is that stopping unwanted sounds from outside of your room. It also keeps sounds created inside the room from escaping or from being picked up in the studio room next door.
Acoustic Treatment
Acoustic Treatment is a set of wall and ceiling panels that absorb and diffuse noises bouncing around that are generated inside of your room.
The term 'acoustic treatment' refers to the insulation batting inside the walls and the rigid fiber glass panels hung about the room that help absorb excess sound reflections in the room. If you clap your hands loudly in an empty room you'll hear a spring reverb if the room is small and if it's larger you'll hear an echo.
Acoustic treatment stops the sound from bouncing around by absorbing it and turning the vibrations into heat. This is also a crucial part of achieving a perfect-quality but natural sounding recording.