Why YSK: Your signals alert other drivers as to what you’re doing; a signal bulb costs a few bucks and is usually a quick and easy repair to do yourself (consult YouTube); and any place that regulates motor vehicles probably requires you to have working turn signals. So knowing when and how to replace a burned out signal bulb can save you an interaction with law enforcement.
Adding: You can diagnose which bulb is out by turning on your hazard lights and checking all four corners of your car. It’ll be the one not flashing.
This is also probably a good time to check your brake lights. Put something heavy on the pedal or have a friend hold it down and check that all three brake lights illuminate. Replacing a burned out brake light is also usually pretty cheap, quick, and easy.
You should also know that (most?) Auto Parts stores will either loan you a screwdriver to change the bulb, or actually do it for you. For free.
Be prepared, though. It’s easier in some vehicles than others. Some will require you to move other things out of the way first just to get to the bulb. The bright side is these days there’s a YouTube video for everything.