The words neap, neep sound the same but have different meanings and spellings. Why do neap, neep sound the same even though they are completely different words?
The answer is simple: neap, neep are homophones of the English language.
The tongue or pole of a cart or other vehicle drawn by two animals.
A neap tide.
Designating a tide which occurs just after the first and third quarters of the moon, when there is least difference between high tide and low tide.
To trap a ship (or ship and crew) in water too shallow to move, due to the smaller tidal range occurring in a period of neap tides.
The swede (rutabaga), called "turnip" in Scotland.
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License 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").