The words filets, fillets sound the same but have different meanings and spellings. Why do filets, fillets sound the same even though they are completely different words?
The answer is simple: filets, fillets are homophones of the English language.
Plural form of filet.
A narrow strip of ribbon or similar material, often worn as a headband.
A strip or compact piece of boneless meat or fish, especially the beef tenderloin.
A boneless strip of meat rolled and tied, as for roasting.
Architecture A thin flat molding used as separation between or ornamentation for larger moldings.
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").