A property is a set of Google Analytics reports and data for your website and/or app. The reports and user interface that you see vary according to what kind of property you have selected to view at the moment. For example, a Universal Analytics property has different reports than a Google Analytics 4 property.
For a website, you can create a Google Analytics 4 property or a Universal Analytics property, or both. Creating both allows you to view both Universal Analytics reports and Google Analytics 4 reports. You can switch back and forth between the two different kinds of reports via the property selector or Admin screen.