What does FKK stand for? I looked it up and got lists of nude beaches, but no definition for FKK.
The German abbreviation means Freie Koerper Kultur (Free Body Culture as already answered) and nowadays is used to denote nudism/naturalism as practiced in public or at least with other people.
It began to be widely propagated in Germany around the end of the 19th century and early 20th as a way of promoting a healthy lifestyle as opposed to the prevalent urban bourgeois/proletarian lifestyles of the times in which living in dark, curtained houses was considered the norm, or else of laboring in dark, crowded, filthy factories or workplaces with even more dilapidated housing for the working classes.
Nudism was a way to get people of all classes into the open air and the countryside or beach, promote exercise, sports, swimming and hygiene in general. The movement was quite a success in Germany but the aims and philosophies were often different from one FKK organization to the next.
An FKK group might for instance be more interested in promoting collective nude activities as a means of enhancing social cohesion amongst group members - think of this as a more socialist way of approaching nudism. Other groups were more interested in promoting healthy living in general and often involved themselves in promoting specific diets in addition to nudist sports activities. These last were sometimes referred to in Germany as 'Reform' groups/philosophies. The Reform movement also ran a chain of health food stores selling items as whole grain bread and various types of herbal teas and other foods considered healthy. I remember that even in Belgium in the 1960s, Reform stores were the main shops where real whole grain bread was available.
Other FKK groups might also be inspired by psychological theories concerning sexual repression, promoting the shedding of neuroses (mostly sexual or about nudity). These groups were not always well thought of by other more traditional and chaste FKK groups, as the focus on sexuality - always a natural enough assumption with nudism - gave the FKK movement a bad name in some circles.
And then again at later dates - 1920s-1930s - other German FKK groups were focused on racial purity and cultivating the ideal (Germanic) body. For this last way of approaching FKK, think of Leni Riefenstahl's movie about the 1936 Olympic games called 'Olympia, Fest der Schoenheit'. In these times it was a short step from FKK to promoting the virtues of a perfectly built and racially superior human, proud to show off health and vigor while hinting at sexual fecundity.
It's a fascinating subject, especially as it mixes up all manner of philosophies and ways of approaching and incorporating sexuality along with all the rest.
The post war FKK movement is now more or less free of all the encumbering philosophical baggage that characterized its beginnings.