The words massed, mast sound the same but have different meanings and spellings. Why do massed, mast sound the same even though they are completely different words?
The answer is simple: massed, mast are homophones of the English language.
Simple past tense and past participle of mass.
Nautical A tall vertical spar, sometimes sectioned, that rises from the keel or deck of a sailing vessel to support the sails and the standing and running rigging.
A vertical pole.
A tall vertical antenna, as for a radio.
A captain's mast.
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.