Nextjs Make it possible to achive Static Generation and Server-side Rendering. The difference is in when it generates the HTML for a page.
Static Generation is the pre-rendering method that generates the HTML at build time. The pre-rendered HTML is then reused on each request.
Server-side Rendering is the pre-rendering method that generates the HTML on each request.
Importantly, Next.js lets you choose which pre-rendering form to use for each page. You can create a "hybrid" Next.js app by using Static Generation for most pages and using Server-side Rendering for others.
Most clearly you can check out the docs to know more.