Here's a simple explanation for what just happened to the stock market

Why is this happening, and should you be worried?

In terms of worry, that's really up to you.
In general, it's not yet time to panic. The overall US economy is still sound. The housing market is pretty strong. Continue to add jobs. US economy is growing, albeit at a slow but steady!

So, what's going on?
There is always a variety of factors involved global markets, but the main thing people are looking at is China.

China's stock market, the Shanghai Composite, started tanking in mid-June, and it is down ~40% from its peak.

China has tried, in multiple ways, to prop up its stock market. Nothing has worked.

Then in August, China devalued its currency, the yuan.
But, in short: The US dollar was getting strong relative to the yuan, which was making China's exports more expensive. That was risky for an economy that was already slowing down and missing expectations. So to make its exports cheaper, China moved to make the yuan cheaper.

China's tinkering with markets freaked investors out, with the read-through being that if the government has to take unprecedented, extraordinary measures, it usually means things are really bad. Or at least worse than the Chinese government was letting on.