Using for...in

Can anyone tell me how for…in works? I know, I know there are dozens of tuts online but for…in always eludes me. Let me have it straight if it’s ok. Here’s what I know, in

for(i in obj){
    //code
}

What I don’t know is how does “i” fit in all this and I heard that it’s always in a string? I don’t get it.