The ability to learn fast and don't be afraid to ask questions. You only have 12-weeks to finish a medium sized project. During which you need to learn about the existing code base, understand how your project fit into the big system, and implement the change. Note the the project will never be as straight forward as a homework project. Unexpected things will slow you down a lot of times. You will also notice other area of improvement to the current system which I encourage you to fix if you can. For example, my intern project was to optimize the pipeline that builds the search index for indeed. While testing my pipeline, I realize that the existing integration test builds the full index, which is unnecessary and takes a lot of time (~4 hours per test). I spent half a week and updated the test to based on a sampled index which makes the test 10x faster and just as reliable.
As a software engineering intern one of the biggest skills is effectively asking questions. At a large company systems are so complex that even new experienced engineers have trouble getting up to speed. You should spend time learning yourself, but also train in asking concise questions to get help from others around you.