Interesting idea. What he's saying is that instead of rendering your single page webapp entirely on the client-side like React or have the user wait for the who

Interesting idea. What he's saying is that instead of rendering your single page webapp entirely on the client-side like React or have the user wait for the whole thing to be generated and sent over with server-side rendering, you can carve up the page into sections (islands) based on whether they contain more or less interactive/dynamic content. Based on these sections, less interactive stuff like text can be rendered on the server and sent over while more dynamic stuff is in skeletal form until client side rendering occurs. This allows the user to see the most important stuff under the correct link first while waiting for any dynamic components to be loaded client-side. Definitely works for some types of SPAs, maybe product sites or social media, but for more complex webapps like scientific tools or presentation software this might not be the best fit.