The Machi is the small gap between the tang and the main part of the knife. The term was taken from Japanese sword makers that gapped their swords to allow for a tsuba to protect the fingers and small parts called seppa that are put on both sides of the tsuba to keep it from moving.
This gap was adopted as an aesthetic part of Japanese kitchen knives and it was preferred in the Tokyo area mostly, so you see this as a standard feature of knives from and sold to that area like the Masamoto kk series we offer. Many pro cooks in other areas prefer the handle to be butted up against the machi so there is no gap. Customers in the USA usually prefer it this way too since they tend to think this is a flaw in the handle installation (which it’s not). I've gotten many questions about this over the years when customers buy a knife and they get one with this little gap so we request them jammed in flush when possible.
Here is a picture of the machi: