I found "su" is also mean "your", what use it for?
Is it a formal for your?

Question

I found "su" is also mean "your", what use it for?
Is it a formal for your?

Answer

Su: 
1.-.Third person of possesive . ("Yo fuí a su casa" it's mean "a la casa de ella", o de él).  Or "Yo soy su empleado" = I am his employee
2.- Also....Second person of possesive, in an formal treat, respect...when we  use "usted" instead of "tú"
I am your employee = (with respect= "yo soy su empleado" ) 
                                   =(with friendship= "yo soy tu empleado")

Answer

True, as Juan Carlos said, "su" means "your" in active phrases to express respect and good manners....

Answer

We use it to express that one thing belongs to one person (third person of singular and plural.
Examples:
Ella es Marta. Su casa es azul.
Ellos son Ana y Juan. Su casa es roja.

Answer

"Su" is a third singular person´s possessive adjetive . We use it to express that one thing belongs to one person (third person of singular, masculine or femenine). Example:

Yo rompí "su" juguete--> I broke his/her toy.
Esta es su casa--> This is his/her home.