whats the difference between 256 kbps and 128 kbps?

Regan Ron,

Those numbers refer to the encoding rate of the file.

The size of a 256 file is about 2 MB per minute of music, while the size of a 128 file is about 1 MB per minute of music.

Assuming that the encoding started from a good quality master, the 256 file technically has better audio quality. However, depending on your ears, equipment, and listening conditions, you may or may not actually notice a difference.

Nowadays, 256 is the most commonly used.