Rekeying is changing the internal combination of a lock cylinder so that it opens with new keys and the old ones no longer work, without replacing the whole cylinder.
Some cylinders are designed for it and are rekeyed with a tool or a special key; with others you would have to take them apart and change their pins. With most ordinary cylinders it is not worth it: fitting a new one is simpler and more reliable.
It makes sense when keys are lost, when tenants change or when you buy a resale property, just like changing the cylinder. If you send us a photo of yours on WhatsApp, we will tell you what you have. In most cases the answer is lock cylinder replacement, with a fixed quote before we start.