Knock Knock \ Who’s there? \ Urine

Knock Knock
Who’s there?
Urine
Urine Who?
URINEsecure don’t know what for

This is a pun, and also a mass culture reference. The pun is centered around the words "urine secure", which sounds like "you're insecure".

The punchline is a reference to the song "What makes you beautiful", which isn't really very funny. I never understood why a knock-knock joke about a song is considered to be funny.