A miter (or mitre, if you’re British) is when you make a corner by joining two end pieces of wood (or tile or stone or whatever material) that are shaved off at a diagonal. Alternatively, refer to the graphic we’ve provided for you, because a picture is worth at least this paragraph of words:
[do you have a graphic of countertop edge joints? I would crop a miter one and then a butt/square joint and bullnose joint for comparison]What is great about this joint is that it can minimize the appearance of seams and create a cleaner look. In countertops, especially light-colored materials, seams will be less obviously visible with a miter than other joints like a bullnose or butt joint. If your material has veining, with a miter cut, you can also visually wrap that veining around the edge like so:
(need high res version from Sunset Cliffs project)
As a side note, a miter can also be a religious headdress—but don’t worry. It is unlikely that people in the remodeling industry will confuse the two meanings.