As the manual states:
Upon opening, files of supported filetypes are parsed to extract the
symbol information (aka "workspace symbols"). You can also have Geany
automatically load external files containing the symbol information
(aka "global tags files") upon startup, or manually using Tools -->
Load Tags File.
Global tags files are used to provide symbols for autocompletion and
calltips without having to open the source files containing these
symbols. This is intended for library APIs, as the tags file only has
to be updated when you upgrade the library.
Some global tags files are distributed with Geany and will be loaded
automatically when the corresponding filetype is first used. Currently
this includes global tags files for these languages: C, Pascal, PHP,
HTML, LaTeX, Python
For other languages, like Java, you have to download a tag file from here (if available), and load it upon startup or manually.