Is it ensure, insure, or assure?
late 14c., from Old French coment "commentary" or directly from Late Latin commentum "comment, interpretation," in classical Latin "invention, fabrication, fiction," neuter past participle of comminisci "to contrive, devise," from com-, intensive prefix (see com-), + base of meminisse "to remember," related to mens (genitive mentis) "mind" (see mind (n.)). The Latin word meaning "something invented" was taken by Isidore and other Christian theologians for "interpretation, annotation." No comment as a stock refusal to answer a journalist's question is first recorded 1950, from Truman's White House press secretary, Charles Ross.
early 15c., from Middle French commenter (15c.), from Latin commentari, from commentum (see comment (n.)). Related: Commented; commenting.
(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 code with too many comments is also bad, 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. Comments that explain __why__ something is done and how the code relates to its environment are useful.
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);