Original: The original string theories from the 1980s describe special cases of M-theory where the eleventh dimension is a very small circle or a line, and if these formulations are considered as fundamental, then string theory requires ten dimensions.
Simplified: The first string theories from the 1980s describe special cases of M-theory where the eleventh dimension is a very small circle or a line, and if these ideas are important to string theory, then string theory requires ten dimensions.