For information about what Recoll does, see its Home Page
Before starting to index and search, you should probably also take a look at the Documentation.
The package ships a user systemd service and timer for per-user index creation and updating.
To enable scheduled indexing, do as user:
systemctl --user enable recollindexuled.timer
systemctl --user start recollindex.timer
The timer is configured to run twice a day at six, and one hour after a reboot.
To just run the indexer once or whenerver needed, do
systemctl --user start recollindex.service
At first run, that will create necessary configuration directories at
~/.recoll. The default config has been tuned a bit for Sailfish OS, and you
should review ~/.recoll/recoll.conf and adjust to your needs.
You should primarily make the decision whether to index things like ~/.qmf
where parts of emails are stored, or ~/android_support, where user data of
Android Support is kept.