It's always big news when a previously unknown work by a major composer is unearthed -- especially if that composer is Mozart. In this case, it was a short Allegro for piano, composed around 1766-1767 by a very young Mozart.
According to the Mozarteum Salzburg Foundation, the work was discovered in a 1780 notebook kept by a village music teacher. The book contained a variety of keyboard compositions, including some by Mozart's father, Leopold (the more prominent composer of the two at the time).
Once scholars verified the authenticity of the work, the Foundation began planning the world premiere. On March 23, 2012, Florian Birsak performed the Allegro in Mozart's Salzburg home, on a fortepiano owned by Mozart -- now that's authenticity!
Realistically, the work's a minor composition. The Allegro isn't a lost masterwork -- it's not likely to join or supplant any of the core repertoire of Mozart piano works. It doesn't foreshadow or provide insight into any of the composer's major compositions. It's simply a very pleasant and tuneful little piano piece. With the name Mozart attached to it.
And perhaps that's enough.