new unfinished cabinets in bathroom
Most modern cabinets, the middle and higher end ones anyway, are made from solid wood stock. The face frames and doors are solid wood while the carcass walls are plywood or veneer. If they are left un-sealed the raw wood will eventually degrade in a moist environment and become smudged with grime and oils.  Either way starts with sanding. Progress to a finer final grit and wipe down with a tack cloth or a rag dampened in mineral spirits. Stain if desired and let stand 12-24 hours.   Now for the seal coat which protects the wood. You have a few choices, ranging from a lacquer sealer to a polyurethane.  Go with a urethane sealer that is brushed on and apply 3-4 coats letting each dry and sanding between each application. For a bathroom you might consider a marine grade varnish which better protects against moisture.