How do you know which order to play stacked notes in

1. How do you play stacked notes?
2. What does it mean when notes are stacked on top of each other?

How do you play stacked notes?

When two notes appear stacked, or attached to the same stem, you play them at the same time. You know, in harmony. Add intervals to the right hand under a melody line. Play intervals in the left hand while the right hand continues the melody.

What does it mean when notes are stacked on top of each other?

And yes, all the notes that are vertically stacked including both staffs - are to be played simultaneously. The notes on the top staff form a chord and the note played at the same time is a corresponding bass note.