I assume you mean this:
sizeof (ledPins) / sizeof (ledPins [0])
bit? (the rest is just a plain "for" loop).
"sizeof" returns the number of bytes in its argument.
"ledPins" is an array of "int"s, in this case there are three of them, and on the Arduino, an "int" is two bytes long, so "sizeof(ledPins)" returns 6.
Now "ledPins[0]" is the first element of the array "ledPins", so it is an "int", so "sizeof (ledPins[0])" returns 2.
6 / 2 = 3, the number of elements in "ledPins".
Imagine you'd ported your code to an architecture where an "int" is 32 bits (four bytes) long. The operation still returns 3.