Via @benschwarz:
Axel tries to accelerate the downloading process by using multiple connections for one file, similar to DownThemAll and other famous programs. It can also use multiple mirrors for one download.
Getting sudden flashbacks to this old fart here:
Think it was 1999 or 2000 when I first use FlashGet (then named JetCar) to more efficiently use my 56kbps dial-up connection (where you had to pay by the minute, not megabyte) 😃