Yes boring old pure data and programming algorithms. This skill is more useful then learning a dozen frameworks. In interviews you'll have people asking you dozens of questions that will all boil down to, "Do you know algorithms?" And not to mention that toolset is applicable over nearly every development domain. You'll very very rarely find software teams who will scoff at somebody who knows their algorithms. You'll rarely find times in your developer life time where you'll think, "Jeeze I wish I didn't know how trees, bloom filters, hash based indexing, etc. works this has been such a determent to my career!"
Most fields of development end up needing to use algorithms at some time or another. How does your data analysis work, How is machine learning done? What are databases doing to make 10,000 queries per second? What are the pros and cons? Are you entering data in inefficient ways to slow this down? Are you or the tool the problem?
When you learn algorithms you can answer these questions. Then apply those answers to your job! Impress your friends and enemies! Demonstrate counting sort running circles around their quick sort implementations because your data is highly specialized.
Yes it isn't sexy. Its not a buzz word. Its horn rim glasses, dnd nerd boring stuff. But at the same time it isn't. Rock-stars learn music theory one way or another, how are you gonna audition for the rolling stones if you don't know what an G cord is?