15-251 Great Theoretical Ideas in CS is the freshman spring semester discrete math & theoretical CS (a follow up to the discrete math course they take freshman fall). The course itself goes for breadth over depth. In this aspect, it serves as a solid foundation for all theory classes students take going forward. It's a class where students learn proof techniques, collaboration, and problem solving.
The course is often praised for its rigor. The problem sets require a fair amount of work to complete, and the course staff sets a high standard for responses.
On the other hand, it commonly receives criticism for being quite intense as a freshman class. Many CMU students look back on it as one of the hardest classes they took.
I personally feel quite lucky to have taken 251. It gave me a foundation to appreciate a lot of deeper CS topics, and taking it freshman year meant I had 3 years more to put it to use.