Besides being a letter in the alphabet, C is also one of the world’s most popular programming languages. (For info on what a programming language is, see How does a computer understand code?)
Before C, the top programming languages included COBOL and BASIC. These languages were supposed to be easily read by people who weren’t fluent in it. As a direct consequence, these languages were very inefficient and unfriendly to the people using them.
Then C came along. It was everything a ’70s programmer (or any programmer) could want:
C is especially noted for its ability to manipulate memory, much like assembly language (see this for more info). This meant that C combined the power of assembly with the portability (able to be run on multiple different machines with little effort) of high-level programming languages.
So, as you can “C”, C and C++ revolutionized programming and quickly supplanted COBOL, BASIC, and other less efficient languages.
Bonus: Why the name “C”?
It all started with the Combined Programming Language (CPL), a not-so-successful language which is C’s earliest ancestor.
CPL was scaled down into BCPL (Basic CPL), which was intended for writing other programming languages. It was also not successful.
Then came along B, a derivative of BCPL. It was designed with portability in mind, and was meant to develop system-independent apps. What is crucial about B is that it was developed in the same place as C: Bell Labs.
C was created in order to re-write the UNIX operating system in an efficient manner. While it was planned to be done in B, several features of the computer which UNIX was supposed to run on simply didn’t work with B. C was created to address these issues with B.