It's a specialized tee. It has one side that accepts the end of the high pressure fuel line that goes in to the fuel rail, one side that mimics the end of the high pressure line and goes into the fuel rail in place of it, and one that accepts a pressure gauge. It bolts into the fuel rail with longer versions of the same bolts that hold the high pressure line in place, and seals with the same kind of orings.

Not responsible for advice not taken...