Hi William, the old Shields only had a capacity of 8 rounds. The Shield Plus uses a double stack magazine and has upgraded capacity. It also boasts several upgrades, especially in areas like the trigger guard and the material composition, including the inclusion of stainless steel components.
Smith & Wesson significantly improved upon the trigger design to offer a lighter, crisper pull, giving the user a smoother and more consistent shooting experience.
Stay safe & have a great day!

the shield plus is a double is a doublestack mag the shield is a single stack

Besides what James mentioned in terns of magazine size & round capacity being higher. Here is a great article comparing the two models.
https://overwatchprecision.com/blog/mp-shield-vs-shield-plus-an-overview-/

Hi William and welcome. A Shield plus uses higher capacity double stack magazines instead of the regular Shield's single stack magazines. Cheers and stay safe!