Since you need the joke explained to you:
- George has an undesirable personal quality that he lacks self-awareness about. It's being brought up in such a way that makes him feel defensive and self conscious. He turns to Kramer for support, but Kramer confirms Jerry's initial assertion. You could replace "chucking the ball" with literally anything negative. We have all experienced this moment. This is one of the tenets of "observational humour", a style Seinfeld is known for. The audience member draws a parallel between the situation on the screen and their own situation. Although this is a common situation, it is rarely spoken about, and so by speaking about it to an audience, it forces an examination of ourselves.
- The fact that the specific thing being discussed is irrelevant adds a layer of farce to the original observation.
- There's also a commentary being made on how people create these domains of specific knowledge. Like, no one learns that there's such a thing called "a chucker", but evidently people think about this situation enough that they come up with a little term to refer to the thing. All three of them know what he's talking about and accept the word, despite the fact that he just invented the word off the top of his head. In many cases, observational humour works not just because of how it makes us reflect, but because it provides us a lens through which we can better express feelings we already had. In this case, the lens is the word "chucker", which describes something we know about but didn't previously have a word for.
- There is verbal humour involving the word salad the characters are speaking. By repeating words quickly, it evokes tongue twisters or riddles. It's humourous to watch the actors speak in such a way just as it would be in real life. Either because they are seemingly unaware they are saying the same word over and over again (again, observational humour about a lack of self awareness) or because they are aware, but are unable or unwilling to desist.
Regular human beings, by the way, don't need this explanation, because the joke is so self-evident and the themes it's touching are so obviously ingrained into basic human behaviour. It's not the funniest joke in the world, or in the series, or probably even in the episode, but it doesn't take a brain surgeon to be qualified to figure out what the joke is.
