Here's why:
* Son means "it is" (plural because we're talking about the time).
* las means "the" (feminine plural because "once" is feminine).
* once means "eleven".
* y means "and".
* cuarto means "quarter" (referring to a quarter past the hour).
You can also say "Son las once quince", which literally means "it is eleven fifteen". However, "Son las once y cuarto" is more common and natural-sounding in Spanish.