This is the 7775th most frequent Polish word.
marsza
"March" (musical or military movement).
Here, 'marsza' is in genitive form, used to indicate 'a moment of' the march.
Po krótkiej chwili marsza, zaczęli tańczyć.
After a brief moment of the march, they started dancing.
In this context, 'marsza' is in accusative form as the direct object of the action 'zagraj'.
Proszę, zagraj marsza na pianinie.
Please, play the march on the piano.
The word 'marsza' is used in its accusative form, showing it as the direct object of the verb 'grają'.