It’s named AltSoftSerial because it is a software-based serial library that is an alternative to the NewSoftSerial package.

If you've got an Arduino of any variety, you might have noticed some of the pins on the board have a tilde mark or PWM printed next to them. What is PWM?

It's of course backwards compatible with existing Arduino add-ons, but sets itself apart with a new USB controller with lower latency and driver-free operation on Mac and Linux.