When we talk about digital audio, let's first understand that CD-quality -- and all MP3s -- are in 16-bit . Bit depth is the amount of bits in a given cross-section of digital audio.
The greater the bits, the higher quality your audio will be. The standard for high-resoution, high-definition audio is 24-bit; the standard for anything considered "standard" digital audio is 16-bit.
To give you an idea of the difference in quality, the same recording, recorded in 16-bit vs. 24-bit, will be one-third the size when in 16-bit as compared to 24-bit. However, the resolution will be greatly increased.