233 is a Internet "word" in China.It means "laughing",just like "lol" or ":D".
It is from the No.233 emoticon of Mop forum which is one of the biggest forum in China and that emoticon is a laughing figure.
And Chinese people on the Internet also use 23333 or 233333333333333 to express the happy emotion.
That movie is very funny23333333
Also,Japanese people on the Internet prefer to use wwwwww to express laughing.
That movie is very funny wwwwww