What's a fonds?

Good question.

Here's what Wikipedia says:

The term fonds originated in French archival practice, but has now spread to English-speaking countries as well. In some countries, including the United Kingdom, Australia and Canada, it has officially replaced the misleading term collection, which used to be used for this level and is now only used for document aggregations assembled, but not created, by a collector.

As Defined by the Society of American Archivists:

Fonds
  n. ~ The entire body of records of an organization, family, or individual that have been created and accumulated as the result of an organic process reflecting the functions of the creator.