The filter prefix is used to automatically append a short text string to the beginning of the filter label name. Filter prefixes are attributes of filter groups and filter subcategories.
For example, suppose my filter group is called "Locations" and I have a filter inside that group with the human-readable name "Botswana" and the label "botswana". If I edit the filter group to have the prefix "geo" then my filter "Botswana" will have its label name changed to "geo__botswana".
Filter prefixes are a helpful way to organize the label names of filters, making them easier to find when typing and for distinguishing filters that have the same name but different meanings (homographs).