Specifically the Model Identifier.
Go to the Apple Menu, top left of the screen, and choose “About This Mac”
At the window that opens, choose “System Report” and then, in the left pane, click Hardware.
In the Hardware Overview, note the “Model Identifier”
It will be something along the lines of “iMac18,3”
Your number may be different. The word iMac is obviously only applicable if you have an iMac, any other model of Mac will show its model name before the two number.