public void SetSiblingIndex(int index);

Public is an access modifier. Keep this public if you want other things to be able to access it. Private if not.

void means that this function does a thing but does not return a value after being executed.

(int index) tells you that in order to use this function, you pass a parameter to it that is an integer.

The name of the function being SetSiblingIndex tells you that this function is used for setting / changing / assigning the index value of a sibling item. In other words, this will allow you to shuffle the order of a sibling object.

Transform.SetSiblingIndex

The fact that SetSiblingIndex comes after the dot after transform tells you that this is a member of the Transform class. Because it is used with a (), that tells you that this member is a function or a method.

Granted, you could just click and drag the order of items in the hierarchy manually, which is what you'll do 99.9% of the time. But for those rare use cases where you need to use a script to change the index of a sibling, there's a function for that. The function's documentation may not be immediately readable to a lay person, but by the time you've gotten a good understanding of OOP, you can more or less discern what this is and how it might be used.