The words main, mane sound the same but have different meanings and spellings. Why do main, mane sound the same even though they are completely different words?
The answer is simple: main, mane are homophones of the English language.
Most important; principal. See Synonyms at chief.
Exerted to the utmost; sheer: by main strength.
Nautical Connected to or located near the mainmast: a main skysail.
Grammar Of, relating to, or being the principal clause or verb of a complex sentence.
The long hair along the top and sides of the neck of certain mammals, such as the horse and the male lion.
A long thick growth of hair on a person's head.
Definitions 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").