The words instance, instants sound the same but have different meanings and spellings. Why do instance, instants sound the same even though they are completely different words?
The answer is simple: instance, instants are homophones of the English language.
An example that is cited to prove or invalidate a contention or illustrate a point. See Synonyms at example.
A case or an occurrence: In all such instances, let conscience be your guide.
Law A legal proceeding or process; a suit.
A step in a process or series of events: You should apply in the first instance to the personnel manager.
Plural form of instant.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.