The words lean, lien sound the same but have different meanings and spellings. Why do lean, lien sound the same even though they are completely different words?
The answer is simple: lean, lien are homophones of the English language.
To bend or slant away from the vertical.
To incline the weight of the body so as to be supported: leaning against the railing. See Synonyms at slant.
To rely for assistance or support: Lean on me for help.
To have a tendency or preference: a government that leans toward fascism.
The right to take and hold or sell the property of a debtor as security or payment for a debt or duty.
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").