In baseball, a fat pitch is a hittable ball. The odds are in your favor. You might miss, but it is a situation where you should take a swing of the bat. If you swing at good pitches and avoid the crappy ones, you improve your OBP. Once on base, it become a running game.
The stock market serves a lot of curve balls. Now and then comes a Fat Pitch, your odds-on opportunity to swing the bat. Get on base and then manage your base-runners.
Specifically, the Fat Pitch on this site refers to two situations.
First: A Fat Pitch comes at a market turning point. It's an identifiable and quantifiable capitulation point where sellers or buyers have become exhausted and panic or euphoria is at an extreme. The Fat Pitch here is measured by a combination of (in no particular order): put-call, Trin, NYMO, sentiment, fund cash balances, major accumulation or distribution, volume, price relative to Bollinger bands, volatility, and consecutive days in a row in one direction. Swinging the bat without throwing up is the hardest part.
Second: The Fat Pitch is a favorable investing environment. Old hands talk about there being only a few good times each year to be involved in the market. The remainder are unprofitable. I think this is correct. The Weekly Market Summary is intended to help discern when its favorable to be long (or short) and when it's best to work on improving your French.
Every day, week and year is a learning experience. The purpose of this site is to help refine what constitutes a Fat Pitch. Like baseball, you have to continue to work on your swing.