What is a rune in Go?

I've been googling but Golang only says in one line: rune is an alias for int32.

But how come integers are used all around like swapping cases?

The following is a function swapcase. What is all the

And why doesn't switch have any arguments?

&& should mean and but what is r

func SwapRune(r rune) rune {
    switch {
    case 'a' 

Most of them are from http://play.golang.org/p/H6wjLZj6lW

func SwapCase(str string) string {
    return strings.Map(SwapRune, str)
}


I understand this is mapping rune to string so that it can return the swapped string. But I do not understand how exactly rune or byte works here.