How does an if statement execute in a for loop? [on hold]

So I watched a tutorial on how to make tetris in javascript. And at the end, he wrote a for loop to check if the row has been filled. Unfortunately, I don't understand how does it work. I would be very please and thankful if someone could explain how the block works. Thank you for reading, this is the loop:

function sweeper(){
 outer: for (y=arena.length-1;y>0; --y){
   for (x=0;x<arena[y].length;++x){
    if(arena[y][x]===0){
    continue outer;
  }
}
   const row = arena.splice(y,1)[0].fill(0);
   arena.unshift(row);
   ++y;
   scoreUpdate();
 }
}