There's a few different uses for this simple but extremely useful piece of equipment.
If you want to make sure your cake, muffins or brownies are done in the oven, insert the Cake Tester's stainless steel wire into the center of the treat. If it comes out clean, the baking is done! No more wasting toothpicks for this simple task.
You can also use it to test the temperature of meats for doneness against the temperature of your skin. For example you'll insert the tester into your steak, then touch it against the bottom of your wrist, if it feels colder, it's under 100F, no difference then right about 100F, and if it feels a good amount warmer it's probably getting close to medium rare, if it feels hot you're probably close to well done. If you can't insert the tester into your meat, it may be overcooked to charcoal and you should throw it away. This is a fun little way to become more proficient with cooking temperatures and increase your skills.
Lastly it can be used to test tenderness and doneness of products, like roasted vegetables, baked potatoes, etc. If the tester enters the cooked vegetable with resistance then they're either still raw or al dente. When the tester enters the cooked vegetables with little resistance it's probably perfectly done.