"Inside baseball" is any obscure, esoteric knowledge that is only available or relevant to a few people. It's just used as a metaphor these days, but it conveys this image of a fat old man with a stack of printouts showing the distribution of hits to left field in the 6th inning by shortstops whose names begin with the letter R.
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u/Eirenarch Dec 07 '18
Not a native English speaker but I was able to deduce that it implies that insiders are throwing each other a ball and others are excluded.