OBD2 scanners are small devices used to access the On-Board Diagnostics of your vehicle, and having one is worthwhile for almost every vehicle owner. It can potentially save you a lot of expensive trips to the repair shop, or at least know how urgent the repair is. The Check Engine light can come on just from not getting your gas cap screwed on all the way, for example. You plug the scanner into your car (usually somewhere under your dashboard) and it can tell you what your car's computer knows about the problem.
Some scanners tell you some details about the problem, and the cheaper ones often just give the error code. Either way, you can google the error code number to get more information, learn about the problem, and make an informed decision on what to do about it.
Most scanners have their own screen to display the error codes and other information, but some have no screen and connect to your phone with Bluetooth.
If you don't want to buy a scanner, major auto parts stores will often scan your codes for free.