What does se te VA mean?

"Se te va" means "it is going to".

For example:

"You are going to lose it" ~ "Se te va a perder".

"You are going to drop it" ~ "Se te va a caer"

Also, sometimes: "Se te va el bus" ~ "The bus is leaving and you are going to miss it"

Just "se te va" also means "you're crazy" in Spanish slang.