You know how sometimes your teacher gives you a test at the end of a unit to see how much you learned? Well, the SAT Subject Tests are kind of like that - they are tests made by a company called the College Board that test you on specific subjects like math, science, or languages.
You usually take them if you want to apply to college, and they can help show colleges how much you know in certain areas. It's kind of like a report card, but instead of getting a letter grade, you get a score from 200-800.
Different colleges may have different requirements for which subject tests you need to take, or they may not require them at all. So, it's a good idea to check with the colleges you're interested in to see if they require or recommend any specific subject tests.