What does 51 50 mean?

The code 5150 is used by the California legal system when someone is designated as a potential danger to themselves or someone else by police, or other professionals such as a mobile crisis team, and can be placed into a Mental Health Facility for up to 72 hours for treatment an evaluation.

It's basically shorthand for health professionals and law enforcement officers to describe someone they feel is suffering from a mental disorder and is therefore subject to a legal involuntary 72 hour psychiatric hold.

In 1986 the rock band Van Halen released their 7th studio album with the Tongue-in-cheek title 5150; this was their first album to feature lead singer Sammy Hagar, who replaced original frontman David Lee Roth.