What does y su pasando una tarde en casa mean in English?

The phrase "y su pasando una tarde en casa" is grammatically incorrect in Spanish. However, based on the context, it seems like you might be trying to say:

"And they are spending an afternoon at home"

Here's a breakdown:

* y: And

* su: They (formal)

* pasando: Spending (present participle of "pasar")

* una tarde: An afternoon

* en casa: At home

Corrected sentence: Y están pasando una tarde en casa.