Gratis means free of charge. Libre means free to use, study, modify, and share. A lot of software is gratis without being libre, meaning you pay nothing, but the app may track & store your data in a format only it can read, and can change the rules or disappear whenever its owner feels like it. Libre flips this, you get the source, you can run it on your own hardware, and you have somewhere to go if the maintainers drift.
Most of what I run is off-the-shelf free software. Open formats, open protocols, no vendor lock-in. Here is a short list that earns its keep: