The words aisle, I'll, isle sound the same but have different meanings and spellings. Why do aisle, I'll, isle sound the same even though they are completely different words?
The answer is simple: aisle, I'll, isle are homophones of the English language.
A part of a church divided laterally from the nave by a row of pillars or columns.
A passageway between rows of seats, as in an auditorium or an airplane.
A passageway for inside traffic, as in a department store, warehouse, or supermarket.
Contraction of I will.
An island, especially a small one.
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.