This is a shortened form of iyon.
Very common in Filipino conversation.
the, that

also spelled as 'yon

Ano 'yun?

What's that?
(something not immediately near)

Yun na nga.
That was it.

Yun ang sinabi ko.
That was what I said.

Normal 'yun.
It's normal.

Be sure to read about ang, iyon and iyan.

Ano ang tawag dito?
What is this called?

Ano 'yung tawag dito?
What's this called?

Ano ang kinain mo?
What did you eat?

Ano 'yung kinain mo?
What was it that you ate?

Ano ang ginawa mo?
What did you do?

Ano 'yung ginawa mo?

What was it that you did?