The words we're, weir sound the same but have different meanings and spellings. Why do we're, weir sound the same even though they are completely different words?
The answer is simple: we're, weir are homophones of the English language.
Contraction of we are.
A fence or wattle placed in a stream to catch or retain fish.
A dam placed across a river or canal to raise or divert the water, as for a millrace, or to regulate or measure the flow.
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").