The words lo, low sound the same but have different meanings and spellings. Why do lo, low sound the same even though they are completely different words?
The answer is simple: lo, low are homophones of the English language.
Used to attract attention or to show surprise.
Of less than usual or average depth; shallow: The river is low.
Having little relative height; not high or tall.
Rising only slightly above surrounding surfaces.
Situated or placed below normal height: a low lighting fixture.
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").