C is said to be the modern assembler language. Unlike real assembler language, C is portable, and C is much easier to write than assembler language.
Like assembler programs, C programs tend to run without slowdowns introduced by the implementations of many other programming language, and, even after 45 years after its initial creation, C remains a popular language for writing system software.
You can find loads of material online about C. I have written some for another course that you might find useful. See Notes on C/C++.