The mainplate is the load-bearing skeleton of a watch movement: the rigid structure on which every other component rests, from jewels to barrel, from gears to escapement. Without a mainplate there is no movement, just a pile of parts. It's the starting point of any caliber, and the quality of its finishing tells you almost everything about who built it.
Materials vary according to manufacturing tradition. Brass is the most common. Maillechort, also called German Silver, is an alloy of copper, nickel and zinc without a gram of real silver: Glashütte manufactures use it, and Minerva historically did, because it works beautifully, holds up to manual finishing and has a very clean visual result once polished. Tungsten carbide is used by Bvlgari on the Octo Finissimo Ultra because it allows extreme rigidity at thicknesses otherwise impossible with brass. Richard Mille created a unibody mainplate in carbon TPT on the RM 27-02: different from everything else, designed with weight in mind.
Finishing on the mainplate is the second language of those who know how to look. Côtes de Genève are wavy parallel lines incised on the surface: they reflect light in a characteristic way and remain the reference of the Geneva school. Glashütte striations are tighter, drier, with a different effect: immediately recognizable once you've seen both. Perlage is a constellation of small overlapping circles, obtained with a rotating tool applied by hand thousands of times: it often covers internal surfaces, those the viewer never sees. Graining works the same way but with steel bristles, creating a more granular matte finish. Radial Côtes de Genève start from a center and expand outward: a more dynamic effect, often used on display casebacks to add depth. Traits tirés finishing is a variant of parallel lines, finer, used by AP on the 2968 caliber. Azurage creates three-dimensional waves on the internal dial. Clous de Paris decoration, visible for example in the Reverso Hybris Mechanica Quadriptyque, is a regular pyramid guilloché, made on a rose engine or by hand.
Skeletonization is something else entirely: it's not a finishing, it's subtraction. Structurally unnecessary material is removed from the mainplate and bridges to make the mechanism visible. It can be done mechanically or completely by hand. Suspended bridges, like in Girard-Perregaux's Three Flying Bridges, are an architectural variant: the mainplate disappears and components are held by cantilevered elements. Harder to balance, more spectacular to look at.
From the bench: a well-finished mainplate has uniform hand-beveled edges, parallel lines without burrs at curves, jewel holes centered without chipping. These are details you only see with a loupe, but they're there, and they distinguish a properly finished movement from one that's simply assembled.