n stands for “nanocores” or “nanoCPU”. Kubernetes CPU metrics are generally “expressed” when a person deals with them in millicores/milliCPU, or 1/1000 of a cpu. a nanocore/nanoCPU is 1/1000000000 (1 billionth) of a cpu. for a little more info the docs have a section on CPU Units but they only really describe millicores/milliCPU.
Ki is a Kibibyte. Kubernetes supports both Binary Units and Metric Units when writing pod specs and such, but for metrics things are emitted as Binary Units.