The words tocsin, toxin sound the same but have different meanings and spellings. Why do tocsin, toxin sound the same even though they are completely different words?
The answer is simple: tocsin, toxin are homophones of the English language.
An alarm sounded on a bell.
A bell used to sound an alarm.
A warning; an omen.
A poisonous substance, especially a protein, that is produced by living cells or organisms and is capable of causing disease when introduced into the body tissues but is often also capable of inducing neutralizing antibodies or antitoxins.
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").