The mortise joint is really the mortise and tenon joint, which is a very sturdy way to join two pieces of wood and is sometimes used in cabinets to join the sections of the frame together.
It is a very old variation (it’s known to have been used for at least the last 4,500 years) of the concept of “insert tab A into slot B”.
The slot cut into one piece of wood is the mortise.
The tab cut out of the other piece of wood is the tenon.
It is an incredibly durable joint, but is typically found only in very high end cabinets, probably because it’s easier to cut circular holes than to cut square holes.
With modern glues, this joint doesn’t necessary need any nails or screws.
It’s that strong.
I hope this information is useful to you. Other details about cabinet construction is available in other articles.