tl;dr Stock and bond prices have an inverse relationship, when one goes up the other goes down.
The Story
Stocks are pieces of ownership of a company and bonds are loans to a company, government, or municipal project. How are they related?
Stocks increase in value when a company’s future has a perceived value higher than today—in other words we expect growth.
Bonds usually pay a fixed rate regardless of a company’s performance.
In “good times” when a company has a great outlook, people would want to own the stocks to take advantage of that growth. Because a bond’s terms were set when it was issued, yesterday’s great rate doesn’t look so good. This increases demand for stocks (price goes up) and lowers demand for bonds (price goes down).
When times are bad or scary, people don’t want stock risk (price goes down) and appreciate the surety of the bond’s payments (price goes up).