What is a showcase piece? I am doing an audition, and they want me to perform a showcase piece. Can somebody tell me the answer?
This is just a general term for something that showcases your skill/ability. They are asking for you to perform a piece that you have mastered very well, and which is challenging enough for it to display how skillful you are exactly.
This might depend on the specific context of your audition requirements, but when I hear the word "showpiece," I think of a medium length or shorter piece of music that was designed to show off the technique and skill of the player, rather than having been composed for the purpose of musical expression (of the composer).
You might hear a showpiece played as a quick one-off on a pops concert, or as an encore to a typical recital.
Violin in particular is closely related to showpieces, due to the immense tradition of over-the-top virtuosity. Some examples might be any of the Paganini caprices, Milstein's Paganiniana, Ernst's transcription of Der Erlkönig for solo violin, Saint-Saëns' Introduction et Rondo Cappriccioso, this ridiculous transcription of Bach's Toccata and Fugue for solo french horn... A common trait is transcribing an established piece of repertoire for a different instrument, forcing the player to jump through a number of technical hoops to make it possible.
Really, the determining factor is just something with a high ratio of "fireworks" (relative to the length of the piece) for the player to execute. I know the pieces I included above are pretty ridiculous in their technical requirements, but showpieces exist at any skill level, so long as they serve the purpose described above.