An inner frame is the articulated skeleton that higher-tier kits build first — a full mechanical body of joints, pistons, and struts that exterior armor panels then attach onto. It is the single biggest difference between entry-tier and enthusiast-tier kits.
Frames reward builders twice: articulation comes from the skeleton rather than the armor, and a painted frame peeking through gaps sells the machine as machinery.
See it in context in our kits & grades guides, or explore the tools hands-on in The Bench.