It's the ratio of the two sides (length and width) to each other. This does not include the plastic or metal strips on either side of the display. It's like hiding your front camera or notch..for particular apps or other settings .
NOTE: It doesn't make any sense..because if hide front camera is enabled..and you set particular app as a full screen mode..and later you want to change the aspect ratio..there is no change.. It totally depends on particular apps and their development..
