what is focusing to infinity?

i've read some about shooting fireworks and the term focusing manually until infinity keeps coming up. i was wondering how this is done, what it does, and what's its purpose? thanks for any help

It means to manually rotate the focus until the 8 (number eight) laying sideways shows up. This moves the focus area of the lens the farthest away from the camera, thus allowing you to take images of the fireworks without having a definite range.
As for its many purposes, I will wait for someone more articulate to step in and explain the actual details, as I have very roughly explained the howto.

Not all lenses are tack sharp at infinity. Mine are slightley to the left. It is however a good reference mark to set the lens to focus on the farthest distant objects on the horizon in low light as a starting point. Your combination of aperture and distance from the fireworks will dictate your focus point. I would still manual focus, then after the first shot, chimp and zoom in on the display to see if it looks good then adjust focus. Better to side on caution since you don't want to come home and find out your photos are all blurred and have to wait till next year to try again.