When you would export your app for macOS from Unity, you might see, that when people on another Macs would try to open that — they would see an alert popup, that will tell them that this is an app from unidentified developer, with ability only to close that popup. Experienced Mac users would know, that to open such an app they would need to right click on .app file, then hold cmd button, and click open. Mac would still prompt them with popup, but now with 2 buttons, 1 of them would allow user to application.
Of course, when you publish your app live, you don’t want your users to meet such things on very first step of their experience with app. To fix that, you need to sign your application with Developer ID certificate.