Can't be opened because it's from an unidentified developer

Unidentified developers are developers not registered with Apple.
Apple has put a warning on macOS apps downloaded from the internet.

"can't be opened because it is from an unidentified developer"

"can't be opened because apple cannot check it for malicious software"

If the app you have downloaded is not in the App Store, then you can override your security settings to run the app.

1. In the Finder, locate the app you want to open.
2. Hold Ctrl and click on the app.
3. In the shortcut menu, click 'Open'.
4. Click 'Open' when the next warning opens up.