The words theirs, there's sound the same but have different meanings and spellings. Why do theirs, there's sound the same even though they are completely different words?
The answer is simple: theirs, there's are homophones of the English language.
Used to indicate the one or ones belonging to them: The red house is theirs. If your car doesn't start, take theirs.
Usage Problem His or hers: brought his own lunch and expected everybody else to bring theirs. See Usage Notes at he1, they.
contraction of there is
contraction of there has
contraction of there are See there're.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, 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.