From LQWiki

A line written within a set of instructions that is not to be interpreted by the compiler or the interpreter. An aside. Mostly used to instruct a human reader about what is happening or to otherwise give guidance within the set of instructions.

In many UNIX config files comments are preceded with the "#" symbol. Though some applications with roots in "C" will use the semicolon ";" for comments (named.conf for instance)

Each programming language has its own set of comment characters and rules about how to use them.