Snake map trouble

I was looking up on how to build a snake game in Javascript and upon looking up some code i came across this function that creates the map for the snake to roam on. I'm not exactly sure how it works but i tested it and it works. Thank you for your time.

function createMap(){ 
document.write("<table>"); 

for(var y = 0; y < height; y++){ 
    document.write("<tr>"); 
    for(var x = 0; x < width; x++){ 
        if(y == 0 || y == height-1 || x == 0 || x == width-1) 
            document.write("<td class='wall' id='"+ x + "-" + y + "'>
</td>"); 
        else 
            document.write("<td class='blank' id='"+ x + "-" + y + "'>
</td>"); 
    } 
    document.write("</tr>"); 
} 

document.write("</table>"); 
document.write("<h1 id='score'>Score: </h1>"); 
}