Purpose: to prepare a version of the field content for use in the search index.
This function should do whatever is needed to return a string of content suitable for searching. For some field types, that may just mean stripping HTML tags, for others it might be selecting the appropriate parts of content and concatenating them into a single string. The map field type, for example, returns the address of the pin, but nothing else.
The string is used for search indexing only, and isn’t displayed to anywhere.