1. [n]  a written explanation or criticism or illustration that is added to a book or other textual material; "he wrote an extended comment on the proposal"
2. [n]  a statement that expresses a personal opinion or belief; "from time to time she contributed a personal comment on his account"
3. [n]  a report (often malicious) about the behavior of other people; "the divorce caused much gossip"
4. [v]  provide interlinear explanations for words or phrases; "He annotated on what his teacher had written"
5. [v]  explain or interpret something
6. [v]  make or write a comment on; "he commented the paper of his colleague"

1. \Com"ment\ (?; 277), v. i. [imp. & p. p. {Commented}; p.
   pr. & vb. n. {Commenting}.] [F. commenter, L. commentari to
   meditate upon, explain, v. intens. of comminisci, commentus,
   to reflect upon, invent; com- + the root of meminisse to
   remember, mens mind. See {Mind}.]
   To make remarks, observations, or criticism; especially, to
   write notes on the works of an author, with a view to
   illustrate his meaning, or to explain particular passages; to
   write annotations; -- often followed by on or upon.
   
         A physician to comment on your malady.   --Shak.
   
         Critics . . . proceed to comment on him. --Dryden.
   
         I must translate and comment.            --Pope.
   
   

2. \Com"ment\, v. t.
   To comment on. [Archaic.] --Fuller.
   
   

3. \Com"ment\, n. [Cf. OF. comment.]
   1. A remark, observation, or criticism; gossip; discourse;
      talk.
   
            Their lavish comment when her name was named.
                                                  --Tennyson.
   
   2. A note or observation intended to explain, illustrate, or
      criticise the meaning of a writing, book, etc.;
      explanation; annotation; exposition.
   
            All the volumes of philosophy, With all their
            comments.                             --Prior.
   
   

(Or "remark") Explanatory text embedded in program source (or less often data) intended to help human readers understand it.

Code completely without comments is often hard to read, but too heavily commented code isn't much better, especially if the comments are not kept up-to-date with changes to the code. Too much commenting may mean that the code is over-complicated. A good rule is to comment everything that needs it but write code that doesn't need much of it.

A particularly irksome form of over-commenting explains exactly what each statement does, even when it is obvious to any reasonably competant programmer, e.g.

        /* Open the input file */
        infd = open(input_file, O_RDONLY);