This is one of Chopin's famous "Twenty Four Preludes." But what are they preludes to? Originally, in the time of Bach, preludes were short, overture-like introductions to collections of dances. The dances were composed to a specific series of rhythms and steps, and the prelude was meant to set the stage, signalling the dancers to prepare to boogie. In this context, the prelude was the performer's only chance to cut loose rhythmically and improvisationally, without having to provide forms and rhythms that the dancers could step to. A prelude could thus take any shape or no shape at all, with rhythms and moods changing according to the whims of the performer. Two hundred years later, the prelude was emancipated from having to precede anything and was considered attractive in its own right! So Chopin strung 24 of them together like a string of pearls, each one in a different mood and key.
Those pearls, by the way, were meant for his lady friend and notorious novelist George Sand. He finished the Preludes on the Spanish island of Majorca, with George and her two children, having fled there from Paris to escape inquiring minds that might wonder why he was being unfaithful to his fiance, Maria Wodzinska. He packed his suitcase with several volumes of J.S. Bach for inspiration. And it just so happens, these twenty four preludes are organized on a Bachian model, covering in sequence every Major and relative Minor key; thus, No. 1 is in C, No. 2 in A minor and on to G, E Minor (this one) D, B Minor, and so on.
