This is one of the most misunderstood things regarding dentures, mostly because it’s not properly discussed,
A  reline is generally done when you have lost weight, or the dentures just need to be fitted more closely. It can be completed while you wait or on a secondary visit, depending on the complexity of your case and the practitioner’s time.  The inside of the denture is slightly removed/reduced and new acrylic  material added, then inserted back into your mouth for a proper current impression.
If you have lost a significant amount of weight, or are certain medications that negatively affect reline materials,  or the denture is either very old and ill fitting or were not fitted well in the beginning, then a rebase is done.  This involves removing the base between you and the chewing teeth and remaking that part  to fit you now, with properly extended borders all around the denture , as well as inside where a reline would have been done.
So, the reline is easier, less time consuming, less expensive, and usually done only once or twice in the life of your dentures, while the rebase is more complex, takes more time, is more expensive, and may help you better than a reline ever would have done.
Again, only after being examined by a properly and experienced practitioner would the correct choice be made for your benefit, sometimes dentures are relines to get you through a period of time until you receive a rebase or new dentures.  A reline is NOT a permanent remedy for poorly made teeth.
Thank you, a VERY good question indeed,