This is an idiom which refers to a discussion about subject involving highly specialized terms and understanding which would only be known to experts in that subject. People not having a deep, experiential knowledge would be excluded.
The term feels like “inside joke,” meaning some people are on the “inside” and others are on the “outside.” But it actually dates to a style of baseball played in the late 1800s that depending on things “inside the diamond” – bunts, stealing bases, smaller base hits, etc – rather than line drives and home runs.
The style became so specialized, that “inside baseball” came to mean a very detailed knowledge of the sport, and it was applied to subjects beyond baseball from there.
Why I Looked It Up
A friend used the term about an industry analyst who often didn’t frame things in terms a non-expert could understand.