Ridge preservation, or sometimes called "socket preservation", is bone grafting at time of extraction to maintain the dimensions of the jaw bone for future use - whether it be for an implant, denture support, esthetics, or simply to keep options of these open.
When a tooth is removed, there is, of course, a hole where the tooth root once was. If this hole is left, it collapses and fills in primarily with soft tissue. In fact, extraction without ridge augmentation typically results in a loss of 50% of ridge width - bone that is essential for implant placement. While ridge augmentation can build this bone back following extraction, it is most predictable and successful when done at the time of extraction, and can save months in treatment time.
It is essential to be evaluated prior to extractions to determine if ridge preservation is indicated in your case.