Bicycles/Maintenance and Repair/Tools and Supplies/Chain Wear Indicator
A chain wear indicator tells you how worn the chain on a bicycle is. As is often believed, chains do not stretch, but rather the rollers become worn. There are various types of chain wear indicators, but they all function by being inserted into the gaps of the chain to determine the amount of chain wear. (If you don't have a chain wear indicator, a ruler can be used.) These devices check to see when 1/16th of an inch of wear has occurred across 12 full links. In general, if the chain is NOT worn out yet, the indicator will not fit in the chain.
After the tires and break pads (which wear out by design), and brake and shifting cables, the part of a bike that needs the most attention is the chain. Because of that and because a chain wear indicator is so easy to use, it should be in the intermediate tool kit, not the advanced tool kit.
If you ride with a stretched chain you will prematurely wear out your chainrings and/or rear cogs (which cost a lot more than a chain). If you let it really go, it can slip. This will of course happen when you are putting a lot of force on the pedals or hitting a bump, and can easily cause a crash.
This chain is worn out