The words rains, reigns, reins sound the same but have different meanings and spellings. Why do rains, reigns, reins sound the same even though they are completely different words?
The answer is simple: rains, reigns, reins are homophones of the English language.
Plural form of rain.
A rainy season.
Third-person singular simple present indicative form of rain.
Plural form of reign.
Third-person singular simple present indicative form of reign.
The kidneys, loins, or lower back.
The seat of the affections and passions.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License, 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.