How does the var x and var lineY code work?

I have this processing.js code made on khan academy:

background(255, 255, 247);
stroke(173, 222, 237);

for (var i = 0; i < 20; i++) {
    
    var lineY = 20 + (i * 20);
    line(0, lineY, 400, lineY);
    
}

for (var j = 0; j < 20; j++) {
    
    var x = 20 + (j * 20);
    line(x, 0, x, 400);

}

What does the

var lineY = 20 + (i * 20);

code do? And what does the

var x = 20 + (j * 20);

code do?

Im a beginner