Normally it means a procedure where a surgeon enters through a smaller opening than a traditional surgery would require. Usually, it's some kind of 'scoped procedure. What the surgeon would do once he enters is another question.
A few years ago I needed a spinal fusion at the base of my lumbar spine. One of the options was a "minimally invasive" procedure, so called because they would go in through a 2-inch opening using a fiber optic scope and cutting tool, instead of using a scalpel and retractor to cut a 6-8 inch flap of skin and muscle on each side of the spine and hold it off to the side. Preserves more muscle tissue and tendon, less scarring. Either way, once they got inside they would do roughly the same thing - scrape out the cartilage between two vertebrae, insert a cage and filler material, then remove a little bone and fix the vertebrae in place with screws and plates before backing out. So, minimally invasive can be kind of a relative term. I don't assume KI needs anything nearly so dire, but it really doesn't tell us much about the recovery time. It sounded like he needed screws out and possibly some other shaving to put a stop to recurring irritation.