Sorry about that. It is groove or slot cut into board or other material to accept another on edge running either across or along the length on the material as opposed to rabbet joint which is along the edge.
Dado[Linked Image from blog.carbideprocessors.com]

When finish it will be similar to this except with the side boards on edge it be stronger with the dado.
I Beam[Linked Image from housatonicrr.com]