Fiche--pronounced like feesh--is French for card, such as an index card. At one time, English speaking people used to use the word in that way. At the same time, lots of information was put onto celluloid cards called microfiche--and, for short, people called these fiche, too.
When I went to university, I worked in the university library and one of my jobs was refiling microfilms and mircrofiche. We had micorfilm readers and microfiche readers in the library. The microfiche were usually stored in long drawers which contained thousands of microfiche and it was picky work putting these all in the proper order according to number. Microfilm were in their own little boxes and relatively easier to file.
The kind of stuff you would find on microfiche would be abstracts and theses, whereas newspapers and journals would more likely be on microfilm. I hope that some of the libraries have kept the readers, because I doubt that a lot of the microfiche have been transferred to digital media.
A few decades ago, the only way for collectors to trade photostatic copies of comic books in an easy way was by putting them onto microfilm or microfiche and sending them through the mail in this way. Instead of sending full-size copies of comics which would be a lot of bulk to mail, you could reduce thousands of comics onto microfiche and send them to others in that way.
There were a few services that did this and this is how a lot of collectors got acccess to comics they couldn't possibly afford in the original.
On CB+, you'll see a lot of comics that were taken from those microfiche copies, usually indicated with an f, and it's easy to spot those pages. The colours are muddy and the lines are out of focus. But sometimes this is the only copy that is available for a comic--and it's better than nothing.