A shank is the bottom portion of a ring that sits under the finger.
When a shank breaks, cracks or is too thin, it’s time to replace or re-shank the ring. This involves cutting off the worn shank and replacing it with one that is thicker and wider.
This provides superior support for the top of the ring and prolongs the overall life of the piece.
This blog gives examples and more information: Re-shanking your rings: What, Why and When.