Comments are useful for education materials, that is documentation, manuals, tutorials, references, etc. becuase they allow in-code, to explain in further detail each successive line(or group of lines), in the syntax and general logic of the application.
Also, how a library works, how the program is supposed to be used or expanded from the user programmer's perspective, etc.
The two types of comments in C are:
/* C classic comment
May span multiple lines*/
// C++ derived comment, endures up until a newline is encountered(up until the end of the line)