The words plains, planes sound the same but have different meanings and spellings. Why do plains, planes sound the same even though they are completely different words?
The answer is simple: plains, planes are homophones of the English language.
Plural form of plain.
Plural form of plane.
Third-person singular simple present indicative form of plane.
Definitions 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.
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").