The words the, thee sound the same but have different meanings and spellings. Why do the, thee sound the same even though they are completely different words?
The answer is simple: the, thee are homophones of the English language.
Used before singular or plural nouns and noun phrases that denote particular, specified persons or things: the baby; the dress I wore.
Used before a noun, and generally stressed, to emphasize one of a group or type as the most outstanding or prominent: considered Lake Shore Drive to be the neighborhood to live in these days.
Used to indicate uniqueness: the Prince of Wales; the moon.
Used before nouns that designate natural phenomena or points of the compass: the weather; a wind from the south.
Used as the object of a preposition.
Used as the direct object of a verb.
Used as the indirect object of a verb.
Used in the nominative as well as the objective case, especially by members of the Society of Friends.
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").