Bitrate is the amount of compression used to encode an MP3. The more compressed the file becomes, the lower the quality, hence a lower bitrate. As an example of what different bitrates will sound like here's a rough guide:

• 16kbps - Telephone
  
• 24kbps - Shortwave Radio
  
• 32kbps - AM Radio
  
• 64kbps - Voice
  
• 96kbps - FM Radio
  
• 128kbps - Tape Cassette
  
• 160kbps - Hi-Fi
  
• 192kbps - CD
  
• 256kbps - Studio

Remember, the less compression the larger the file.