This is the 2nd most frequent Polish word.
na
"Na" in Polish means "on," "at," or "to," depending on context.
In this sentence, 'na' expresses the idea of an activity being performed on an object, in this case, playing on (or at) the piano.
Gram na pianinie.
I play the piano.
Here, 'na' is used to indicate the purpose of the action, specifying the activity 'a walk' as the intended outcome.
Idziemy na spacer.
We are going for a walk.
In this example, 'na' indicates the position of an object, in this context, the book being located on the surface of the table.