code for bullet shooting

[arun]

hi,

i am following phaser tutorial . but, i don't understand the fire: function() code. please help me 

create: function () {
...
this.bullets = [];
// Add an empty sprite group into our game
this.bulletPool = this.add.group();
// Enable physics to the whole sprite group
this.bulletPool.enableBody = true;
this.bulletPool.physicsBodyType = Phaser.Physics.ARCADE;
// Add 100 'bullet' sprites in the group.
// By default this uses the first frame of the sprite sheet and
// sets the initial state as non-existing (i.e. killed/dead)
this.bulletPool.createMultiple(100, 'bullet');
// Sets anchors of all sprites
this.bulletPool.setAll('anchor.x', 0.5);
this.bulletPool.setAll('anchor.y', 0.5);
// Automatically kill the bullet sprites when they go out of bounds
this.bulletPool.setAll('outOfBoundsKill', true);
this.bulletPool.setAll('checkWorldBounds', true);
this.nextShotAt = 0;

 

fire: function() {
if (this.nextShotAt > this.time.now) {
return;
}
if (this.bulletPool.countDead() === 0) {
return;
}
this.nextShotAt = this.time.now + this.shotDelay;

// Find the first dead bullet in the pool
var bullet = this.bulletPool.getFirstExists(false);
// Reset (revive) the sprite and place it in a new location
bullet.reset(this.player.x, this.player.y - 20);
bullet.body.velocity.y = -500;
},