In music, a prelude is a short, inventive, often improv-like piece. To begin with, preludes were always preludes to something, as with Bach's groundbreaking organ preludes, which were often toccatas or fantasias preceding a more severely structured fugue (although, interestingly, many were conceived as independent pieces).
Later composers have titled as preludes both works which precede more complex pieces, and standalone flights of fancy. Some of Chopin's best piano music is a compilation of preludes spanning all major and minor keys (a Romantic's grand gesture in homage to Bach's similarly laid-out Well Tempered Clavier).
A brilliant double CD which interleaves Bach's and Chopin's preludes is Labor Records' Bach, Chopin, The Preludes, (piano, Joao Carlos Martins and Arthur Moreira-Lima). The notes by Eric Salzman provide a thoughtful look at Bach's influence on Chopin.