A spline is a strip of wood, plywood, or other material (such as Masonite) inserted into matching grooves or plows, along the edges of two boards. The purpose is to reinforce and align the edges.
The triangle is just a sample piece. The actual joint was finished earlier today (with its marks to match the pieces), just in time for the inaugural lunch on the island!
Eric getting ready for tomorrow's task: installing the oven/microwave cabinet.