For those without an encyclopædiac knowledge of BWV numbers, 875 is Prelude and Fugue No. 12 from the Well Tempered Klavier; it's one of my favourite prelude and fugue pairs, and would recommend anyone to have a listen.
But, I digress. I'd start by reading around Bach's life to get a feel of the context in which this piece lies. Then, have a look at
http://www.music.qub.ac.uk/~tomita/essay/wtc1.html for some more background information on the set.
Finally, for programme note purposes, for Bach I'd omit most biographical details as he's relatively well known; mention when the WTC was published, and when this piece was written if you can find details; finally, describe the prelude and fugue - mention character, form and so on.