Callback is blocking.. (JavaScript)

I am trying to understand callbacks properly so that I can iterate arrays without blocking the Server.

Please tell me why it is blocking.. what am I doing wrong? I would much rather run this loop, and then do something when it is finished, but not block the rest of the server.

Here is the code:

//callback test

var myID = 666;
var grid = [
    [0,0,0],
    [0,0,0],
    [0,0,0]
];


function find_empty_slot(id,callback){
    console.log("Your ID is" + id);
    callback();       
}

find_empty_slot(myID,function(){
    for(var y = 0, len = grid.length; y < len; y++){
        for(var x = 0; x < grid[y].length; x++){
            if(grid[y][x] === 0){
                console.log("I am a Callback");
            }
        }
    }
});

console.log("This should fire before the Callback?");