what is pygame userevent

import pygame
from pygame import display, font, time, mouse, draw
from pygame.math import Vector2
import random
import sys

class Snake:
    def __init__(self):
        self.body = [Vector2(5, 10), Vector2(6, 10), Vector2(7, 10)]
        self.direction = Vector2(1, 0)
    def draw_snake(self):
        for block in self.body:
            x_pos = int(block.x * cSize)
            y_pos = int(block.y * cSize)
            block_rect = pygame.Rect(x_pos, y_pos, cSize, cSize)
            draw.rect(screen, (183, 111, 122), block_rect)

    def move_snake(self):
        body_copy = self.body[:-1]
        body_copy.insert(0, body_copy[0] + self.direction)
        self.body = body_copy[:]
class Fruit:
    def __init__(self):
        self.x = random.randint(0, cNumber - 1)
        self.y = random.randint(0, cNumber - 1)
        self.pos = Vector2(self.x, self.y)

    def draw_fruit(self):
        fruit_rect = pygame.Rect(int(self.pos.x * cSize), int(self.pos.y * cSize), cSize, cSize)
        draw.rect(screen, (255, 75, 30), fruit_rect)

pygame.init()
clock = time.Clock()

# Game Variables
WIDTH = 800
HEIGHT = 600
FPS = 60
cSize = 36
cNumber = 18

# Game Window
screen = display.set_mode((cNumber * cSize, cNumber * cSize))
display.set_caption('Snake Game in pyGame')

fruit = Fruit()
snake = Snake()

SCREEN_UPDATE = pygame.USEREVENT
time.set_timer(SCREEN_UPDATE, 150)
# Main Game Loop
while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            pygame.quit()
            sys.exit()
        if event.type == pygame.MOUSEBUTTONDOWN:
            pos = mouse.get_pos()
            print(pos)
        if event.type == SCREEN_UPDATE:
            snake.move_snake

    screen.fill((200, 230, 100))
    fruit.draw_fruit()
    snake.draw_snake()
    display.update()
    clock.tick(FPS)