The POSIX standard also defines functions/system calls for file I/O which are commonly found and used on unix systems. You have to remember that in unix everything is a file so these system calls aren't just used for text files but also multiple other things like devices, sockets etc.
To interact with files in C you need to have a FILE pointer a so called stream, which will let the program keep track of the memory address of the file being accessed. In C text files are sequence of characters as lines each ending with a newline (\n). Interestingly you have already been working with file I/O since the begging as C automatically opens 3 files, the standard input (keyboard), standard output and error (both being the display). You have read from and written to these files using scanf() and printf().
Standard I/O