The S&P 500 is like a report card for the US stock market. It's a list of the 500 largest and most successful companies in the US, and it's used to measure how well the overall market is doing. It's like a big ol' basket that holds a little bit of each of these companies, giving you a sense of how they're all doing as a group.
Now, when you invest in the TSP C Fund, you're essentially buying a tiny piece of that basket. This means that when the S&P 500 goes up, the value of your investment in the TSP C Fund will likely go up too. And when the S&P 500 goes down, the value of your investment might go down as well. It's like being a part-owner of a big, diversified portfolio of companies!
