I've heard this term used a lot in literature but I don't understand what the technique is they are referring to. Please help me out here.
According to p.33 of this presentation 'bedding in' is a process of packing the molding sand by ramming the sand around and under the pattern until the sand is tightly packed and even with the parting line. There is also a glossary with foundry and casting terms defined here. This is used when the parts to be cast are quite large, often as a step in pit molding or floor molding. A description of pit molding is found at coursehero.com:
12.20.3 Pit Molding Usually large castings are made in pits instead of drag flasks because of their huge size. In pit molding, the sand under the pattern is rammed by bedding-in process. The walls and the bottom of the pit are usually reinforced with concrete and a layer of coke is laid on the bottom of the pit to enable easy escape of gas. The coke bed is connected to atmosphere through vent pipes which provide an outlet to the gases. One box is generally required to complete the mold, runner, sprue, pouring basin and gates are cut in it.
Floor bedding is discussed in MANUFACTURING PROCESSES By J. P. KAUSHISH. An excerpt from Google Books (also found here)