Determines which version of javascript should be emitted. If you need to use a feature introduced in ES2029, for instance, then use that or higher. No polyfils will be added, though some syntax will be down-leveled: const/let => var if you choose ES5, for instance.
Simply tells the TS compiler to include types for these features. DOM, WebWorker for example. This works with the above "target" setting, allowing you to include types the target doesn't yet support.