Being a newer model, the iPhone 12 is more advanced than the iPhone 11. The iPhone 12 was the first Apple phone to come with 5G, for speedier internet access away from a wi-fi hotspot. It has a better screen than the iPhone 11 (the 12's is OLED, which gives truer colours and better contrast than the 11's LCD panel), and it has a more powerful processor, which makes apps open faster and run smoother.
The camera has been tweaked too, with better low-light shots now the order of the day. And the design has been improved, with its flat edges much easier to grip than the iPhone 11's curved body.
Improvements all round, then.