The words fills, fils sound the same but have different meanings and spellings. Why do fills, fils sound the same even though they are completely different words?
The answer is simple: fills, fils are homophones of the English language.
Third-person singular simple present indicative form of fill.
Plural form of fill. Usually used of multiple people, not of a single person.
Used to distinguish a son from his father when they have the same given name.
See Table at currency.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").