A QR code is simply a representation of a download link plus activation code for your eSIM profile. It is used to easily connect to the eSIM server and download the eSIM profile to your phone.
It is easier to scan a QR code than typing a long string of text, but if for some reason you are unable to scan the QR code, you will have the option to activate your eSIM by entering the required information manually.
Please note that QR codes can be used only once. Once a QR code is scanned and an eSIM profile is downloaded, it cannot be used to download the same eSIM again, whether it is on the same or another device.