If you try to open an app that is not registered with Apple by an identified developer you get a warning dialog. This doesn't necessarily mean that something's wrong with the app. For example, some apps were written before developer ID registration began. However, the app has not been reviewed, and macOS can't check whether the app has been modified or broken since it was released.
A common way to distribute malware is to take an innocent app and insert harmful code into it, and then redistribute the infected app.
The safest approach is to look for a later version of the app from the Mac App Store or look for an alternative app.
To override your security settings and open the app anyway:
In the Finder, locate the app you want to open. Don't use Launchpad to do this. Launchpad doesn't allow you to access the shortcut menu.
Control-click the app icon, then choose Open from the shortcut menu.