The difference between platform-browser-dynamic and platform-browser is the way your Angular app will be compiled.
Using the dynamic platform makes Angular send the just-in-time compiler to the front-end along with your application. Which means your application is being compiled client-side.
On the other hand, using platform-browser leads to an ahead-of-time pre-compiled version of your application being sent to the browser. Which usually means a significantly smaller package is sent to the browser.
The documentation for bootstrapping explains it in more detail.