This Antifeature is applied to apps where the upstream source code is Non-Free in some way.
This does not mean that Non-Free Software is included with the app. Most likely, the F-Droid build has been patched in some way to remove the Non-Free code/libraries, and/or some functionality may be missing. The app's description will usually go into more detail about this.
In our experience, where the upstream developer includes Non-Free libraries, sooner or later they will include more Non-Free libraries, or other anti-features. Frequently they become impossible to maintain/update in F-Droid. If there is an alternative, it is recommended that you avoid applications with this antifeature if future updates are important to you.