Free Drops are regular monthly releases of premium presets that are available for you to claim and keep.
You can find them under the Drop icon in the bottom left corner of the plugin interface.
Inside the menu you'll find the drops for that month, with the ability to download them and save them to your "Liked" presets.
Once you've claimed a preset, you'll see the "Owned" badge appear in your Library. This indicates that the preset is yours to keep forever!
