What should I expect/what should I know about my mother being on a ventilator? I haven't been able to visit her yet.

Discuss w ICU team. The single best thing that you could do to prepare to visit your mother would be to discuss her situation with her bedside nurse before the visit by phone, if psossible who can describe how she is doing etc.. . The situation can be very variable, some people are awake and interactive on a ventilator, others may be deeply sedated, some may have other organ problems that require support etc..