SwiftUI is a framework introduced by Apple in 2019 to build user interfaces for iOS, tvOS, macOS, and watchOS. With it, developers can quickly build applications that will react to any data changes. The developer experience is great and differs quite a lot from UIKit even if you'll still be relying on it in some situations.
SwiftUI is written using a declarative syntax, this way it is possible to simply state what your UI should do.
For instance, a software engineer could write that they want a list of elements consisting of text fields, and then describe the layout, colors & fonts for each field. The resulting code is simpler and clearer to read, which is great because engineers spend more time reading code than writing it!