When you insert a bag in a vacuum machine and close the lid the vacuum pump removes all the air (pulls a vacuum) in the chamber as well as in the bag. After this process the bag gets sealed. After sealing atmospheric pressure enters the chamber and the bag gets pressed down onto the product leaving no air in the bag.
So how much vacuum is needed for a good result?
The vacuum pumps found in vacuum machines are normally rated 0.5 mbar. Theoretically this is then what could be reached inside the chamber. In real life there is always the possibility of small leaks or other factors that will influence the vacuum achieved in the chamber. We normally consider 10 mbar as the border between good and bad vacuum with figures of about 5 mbar being very good.
When gas flushing food, preserve gas is put in the bag after vacuuming and before sealing. This is needed when the product needs to be loose in the bag like sliced biltong or grated cheese.