The C language had only one kind of comment. The comment started with a two-character symbol /* and ended with the symetric two-character symbol */ . Any characters or symbols that were between these two were ignored, because they were removed by the preprocessor.
This kind of comment is usually used for large quantities of commentary, such as at the beginning of programs where the name of the author, the purpose ot the program, etc., are recorded.
In large programs, a portion of code may have several lines removed and replaced by an alternative piece of code. Instead of deleting them, the programmer could just put a /* before and a */ after. This way the code is still there for reference. If the change is unsuccessful then the code may be restored merely by removing the comments. However, as long as it is enclosed in comments, it is ignored by the compiler.
This kind of comment can also be embedded in an expression:
x = y * 2.5 /* proper adjustment factor */ - z
which is functionally the same as
x = y * 2.5 - z