Historically when you looked at replacing full sets of teeth, you would be presented with the options of having dentures. Dentures are a removable option that are custom made to fit your gum/bone ridges. Over time it is natural for your mouth to change shape (due to bone loss) and that your once well fitting denture is needing to be relined.
A reline can be a simple case of adjusting the base of the denture so it fits your mouth accurately again. This will be an on going process as our mouths, bone and gums are constantly changing. How quick this affects us does vary from patient to patient.