Every time you re-compress an MP3, some audio gets thrown away. I opened a FLAC file of this song in Adobe Audition, then saved it as a 128k MP3. I then opened the MP3 I just saved, and re-saved it as a 128k MP3 with a new filename. Repeated that process 300 times.

By the 5th generation, there are noticeable artifacts in the audio quality.

By the 10th generation, it sounds like crap. And the quality only gets worse. The audio sounds progressively worse the more times it's compressed.

It's like playing the game of "Telephone" in which a large group of people line up and the first person whispers something to the second person, who whispers it to the third person and so on. By the time the message gets to the last person, the meaning would have changed.

A similar process happens when an MP3 file gets compressed over and over. Each generation introduces new artifacts in the audio as the decoder imperfectly approximates what audio was thrown away. By the 300th generation, it's hard to understand the lyrics!