Can Someone Explain This AI-Generated Python Game Code?

I asked an AI to generate a piece of Python code for a game inspired by Hades, and I'm not exactly sure what it does. It doesn't work properly in IDLE. Here's the code I got:

```python
import pygame
import random
import sys

# --------- SETTINGS ---------
WIDTH, HEIGHT = 800, 600
FPS = 60
PLAYER_SPEED = 5
BULLET_SPEED = 10
ENEMY_SPEED = 2
ENEMY_SPAWN_RATE = 90 # frames

# --------- INITIALIZE PYGAME ---------
pygame.init()
screen = pygame.display.set_mode((WIDTH, HEIGHT))
pygame.display.set_caption("Hades-Inspired Game")
clock = pygame.time.Clock()

# --------- COLORS ---------
WHITE = (255, 255, 255)
BLACK = (0, 0, 0)
RED = (200, 0, 0)
GREEN = (0, 200, 0)
BLUE = (0, 0, 200)
YELLOW = (255, 255, 0)

# --------- CLASSES ---------
class Player(pygame.sprite.Sprite):
def __init__(self):
super().__init__()
self.image = pygame.Surface((50, 50))
self.image.fill(RED)
self.rect = self.image.get_rect(center=(WIDTH//2, HEIGHT//2))
self.health = 100

def update(self, keys):
if keys[pygame.K_w] and self.rect.top > 0:
self.rect.y -= PLAYER_SPEED
if keys[pygame.K_s] and self.rect.bottom 0:
self.rect.x -= PLAYER_SPEED
if keys[pygame.K_d] and self.rect.right < WIDTH:
self.rect.x += PLAYER_SPEED

class Bullet(pygame.sprite.Sprite):
def __init__(self, x, y, dir_x, dir_y):
super().__init__()
self.image = pygame.Surface((10, 10))
self.image.fill(YELLOW)
self.rect = self.image.get_rect(center=(x, y))
self.dir_x = dir_x
self.dir_y = dir_y

def update(self):
self.rect.x += self.dir_x * BULLET_SPEED
self.rect.y += self.dir_y * BULLET_SPEED
if self.rect.right WIDTH or self.rect.bottom HEIGHT:
self.kill()

class Enemy(pygame.sprite.Sprite):
def __init__(self):
super().__init__()
self.image = pygame.Surface((40, 40))
self.image.fill(BLUE)
self.rect = self.image.get_rect(center=(random.randint(0, WIDTH), random.randint(-100, -40)))
self.speed = ENEMY_SPEED
self.health = 20

def update(self):
self.rect.y += self.speed
if self.rect.top > HEIGHT:
self.kill()

# --------- SPRITE GROUPS ---------
player = Player()
all_sprites = pygame.sprite.Group()
all_sprites.add(player)
bullets = pygame.sprite.Group()
enemies = pygame.sprite.Group()

# --------- GAME LOOP ---------
frame_count = 0
running = True
while running:
clock.tick(FPS)
frame_count += 1

for event in pygame.event.get():
if event.type == pygame.QUIT:
running = False
if event.type == pygame.KEYDOWN:
if event.key == pygame.K_SPACE:
# Shoot bullet towards mouse
mx, my = pygame.mouse.get_pos()
dir_x = mx - player.rect.centerx
dir_y = my - player.rect.centery
length = (dir_x**2 + dir_y**2)**0.5
if length != 0:
dir_x /= length
dir_y /= length
bullet = Bullet(player.rect.centerx, player.rect.centery, dir_x, dir_y)
all_sprites.add(bullet)
bullets.add(bullet)

# Spawn enemies
if frame_count % ENEMY_SPAWN_RATE == 0:
enemy = Enemy()
all_sprites.add(enemy)
enemies.add(enemy)

keys = pygame.key.get_pressed()
all_sprites.update(keys)

# Check collisions
for bullet in bullets:
hit_enemies = pygame.sprite.spritecollide(bullet, enemies, False)
for enemy in hit_enemies:
enemy.health -= 10
bullet.kill()
if enemy.health <= 0:
enemy.kill()

if pygame.sprite.spritecollide(player, enemies, False):
player.health -= 1
if player.health <= 0:
running = False

# Draw everything
screen.fill(BLACK)
all_sprites.draw(screen)

# HUD
font = pygame.font.SysFont(None, 36)
health_text = font.render(f'Health: {player.health}', True, WHITE)
screen.blit(health_text, (10, 10))

pygame.display.flip()

pygame.quit()
sys.exit()
```

Could anyone explain how this works and what the main components are? Any help would be appreciated!