Multimeters are a vital part of any electronics project, but learning how to use one isn't as simple as it looks. This video from Adafruit walks you through pretty much everything you need to know.
The video guides you through the basics of using a multimeter, including what those numbers on the display actually mean, how to pick the right voltage, and how to replace a fuse when you inevitably pick the wrong voltage and blow one. Even if you've been using a multimeter for ages, the video's a nice way to brush up on the basics.
Collin's Lab: Multimeters [YouTube]