What is React JS about?
If you love frontend development, you should have some experience with React JS, this is because it’s one of the greatest frontend libraries out there. Due to its speed, flexibility, performance, etc… a majority of the developer tends to use React JS as for their frontends.
I have an in detailed breakdown about React JS alone, where you can check it out from the link given below.
What is Next JS about?
Next, JS is a wrapper for React JS, which means in other words it is a framework for React JS. Now you will be able to use extra features provided by Next JS on top of all the other features given by React JS. Next JS is basically the icing on top of the cake (React JS).
What is SEO, and how is it useful for a website?
SEO stands for Search Engine Optimization, which means in simple terms, it is the process of ranking your website on Google. SEO decides if your site needs to come on the top of the Google search or the last.
What makes Next JS way more special compared to React JS?
NextJS helps a lot in SEO, by providing better performance than compared to React JS, which means that if you are to develop a website using Next JS over React JS then your website has a higher tendency of getting higher rankings on Google.
So, does that mean that everyone should be switching to Next JS completely ???, not really, it all depends on the website you are building, in case if you are dealing with a lot of API data handling, then it would be a great choice to use Next JS since it provides SSR (Server Side Rendering) whereas React JS doesn’t (Note that using SSR can increase the performance of your website drastically, hence improving SEO), but if it's the case that you are developing a normal static site with no API integrations then it would be ideal to use React because you would be getting the maximum from using Next JS here.
So, in conclusion, it all depends on the requirements of the website you are building, depending on the requirements you can choose which framework fits well!