1. His/Her/Its: This is the most common meaning of "su" and is used as a possessive adjective. For example:
* Su casa - His/Her/Its house
* Su libro - His/Her/Its book
* Su perro - His/Her/Its dog
2. Your (formal): When addressing someone with formal respect, you use "su" to mean "your."
* ¿Cuál es su nombre? - What is your name? (formal)
* Su teléfono está sonando. - Your phone is ringing. (formal)
3. Their: "Su" can also be used to refer to the possessive of "they" or "them."
* Su familia es grande. - Their family is large.
* Su trabajo es muy importante. - Their work is very important.
4. "Of it/them": In some cases, "su" can be used to mean "of it" or "of them" when referring to something previously mentioned.
* El libro. Su autor es muy famoso. - The book. Its author is very famous.
* Los niños. Su madre los espera. - The children. Their mother is waiting for them.
Important Note: "Su" can be tricky because it can mean different things depending on the context. It's always best to consider the entire sentence to understand the meaning of "su."