The most important thing of all is, you need to be passionate about technology, meaning you love build apps and systems regardless of whether it's your day job or not.
Another area is software engineering best practices. You need to know the core best practices, whether using Git properly (git commits, git workflow, pull requests) or automated testing, how to deploy to staging or productions ... etc
The third and last area we look at is soft-skills and that includes your communication and collaboration skills as well as your ability to work in a startup environment.
Keep in mind that you don't need to be proficient in the technologies we currently use. We usually use React for Frontend, but if you're proficient in Vue.js, that's also fine, just be aware that there is a 1-month bootcamp at the beginning of the internship to help you learn the skills you're missing.