Compression?

I have read the archives but still would appreciate an explanation of what a
compressor actually does. Is it a digital signal enhancer or analog
processor? Does it remove the dynamic ups and downs of the volume?
Brian May had built a device in the early Queen days where he'd hit a
button. the playback tape head would move along with the tape until he
hit the button again. So he'd set the period of the echo to a musical
interval (one beat. etc.) then it would be synchronized for
his patented echo solos.

The compressor was originally designed to be an automatic volume control. An
artifact of this function was that it changed the timbre of the input signal
in some musically interesting ways. Some describe this effect as 'thicker'.