Usually I see this as inside (rather than insideR) baseball but it means things that are only interesting to or of importance to a small number of people, not to the majority of people. Most often it is used about political discussion but can be any topic. It is "insiders" discussing something and if you are an "outsider" (95% of people whatever the topic) you won't understand. For instance, on a political talk show the guests discussing the behind-the-scenes political strategy and personalities - this is unknown to and not of interest to most people, only a small group of people who follow politics very closely.

It's a name of the essay about politics

Context, please. It could be a metaphor or an actual term. Either way, I can't help you for sure without know where you saw it.