A trim kit for a microwave is a cosmetic piece that fits around built-in microwaves so that they blend with the kitchen cabinetry, but there is no microwave made by LG that is specifically called a "trim kit microwave." LG does, however, sell microwaves with built-in or optional trim kits.
Trim kits for microwaves can be made from various types of materials and in differing styles. It's possible to get trim built in with a microwave or as an add-on option, and the kits are also available from third-party retailers like The Home Depot. Generally, a trim kit should be chosen based on how well it blends in with the kitchen decor.