“STEM” is the umbrella term to describe science, technology, engineering and maths. In this question, we’re looking for evidence of your passion and interest in the subject outside of formal education. For example, if you want to do study coding, have you worked on any personal projects? Taken part in hackathons? Maybe you’ve taught yourself Python. If you want to study civil engineering then maybe you’ve designed your own suspension bridge? Taken part in an afterschool engineering club or competition?
It doesn’t have to be a big thing, even if it’s learning in your own time or having a family member already in that chosen career who you actively approached for a greater understanding. Show us that you are passionate and have an invested interest in your subject.