It's high-level assembly language. Thus, it allows you to write code with a very small barrier between you and the machine instructions, but with enough of a barrier that you can reuse and organize your code logically. At the same time, it gives you most of the speed and raw bit manipulation of machine code.
The negatives about C:
You're almost as likely to introduce completely silent catastrophic memory corruption with every operation than you are to actually accomplish the task you're trying to do. Most of the things which would normally be bugs are also now major security breaches. C's structure also makes it extremely difficult to do static analysis, making it difficult to find bugs, security vulnerabilities, and make performance optimizations.
How ATS helps:
ATS addresses a lot of these issues by emitting efficient C code. However, at a higher level ATS combines C with an extremely strong type system (dependent typing) which allows you to do things like verify statically that there are no out-of-bound memory accesses through typing. It can do even stronger things like prove your code correct.