Someone's party piece is something that they often do to entertain people, especially at parties, for example singing a particular song or saying a particular poem.
[informal]
party piece in British English
(ˈpɑːtɪ piːs)
noun
informal
something such as a song, recitation or trick that a person often performs at parties
his old party piece of showing the children a bunny fashioned from a handkerchief
Examples of 'party piece' in a sentence
party piece
We save our sympathy for the poor people performing their party pieces in front of millions of viewers.
The Sun (2012)
It's a good party piece to get people guessing what the main ingredient is.