The words flea, flee sound the same but have different meanings and spellings. Why do flea, flee sound the same even though they are completely different words?
The answer is simple: flea, flee are homophones of the English language.
Any of various small, wingless, bloodsucking insects of the order Siphonaptera that have legs adapted for jumping and are parasitic on warm-blooded animals.
Any of various small crustaceans that resemble or move like fleas, such as the water flea.
a flea in (one's) ear An annoying hint or a stinging rebuke.
To run away, as from trouble or danger: fled from the house into the night.
To pass swiftly away; vanish: "of time fleeing beneath him” ( William Faulkner).
To run away from: flee the scene of an accident.
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").