you can also have a look at: https://en.wikipedia.org/wiki/Free
Free may refer to:
- Liberty, also known as freedom
- Free of charge, without payment required
- Gratis versus libre, the distinction between the two meanings above
Even in your provided link to dictionary-cambridge are different meanings listed:
- Not Limited
- No Charge
- Not in Prison
The word which you refer is probably “GRATIS”
Gratis in English is adopted from the various Romance and Germanic languages. It means “free” in the sense that some good or service is supplied without need for payment, even though it may have value.
Compared to propietary Source Code Apps, you are free/allowed to use the Source Code and improve it (e.g.: remove the nag screen). You are even allowed to change the app and resell it again.
The developer has put it under an Apache2 license which grant rights for you, the user. For an explanation of the Apache2 license check out https://choosealicense.com/licenses/apache-2.0/ .
Feel free to donate to any other thing instead, which does not seem to upset you that much.