What is different between 404 error and 403 in SEO?

Obviously, these are completely different error codes so in terms of their meaning will be different.
_ 404 : This error occurs when the user is still able to access the website but they did not find the documentation they needed.
_ 403 : server refuses access requests from the user.
This error is similar to the 401 error, but note the difference between unauthorized and forbidden. In this case no login opportunity was available. This can for example happen if you try to access a (forbidden) directory on a website.

Most people are bound to recognize this one. A 404 error happens when you try to access a resource on a web server (usually a web page) that doesn’t exist. Some reasons for this happening can for example be a broken link, a mistyped URL, or that the webmaster has moved the requested page somewhere else (or deleted it). To counter the ill effect of broken links, some websites set up custom pages for them.
404 Redirect occurs when a customer on your site tries to access a pagename that doesn’t exist. These are dead pages that link nowhere and gives users a bad experience. If these pages remain unfixed, Google will de-index these pages and all the valuable links, rank, SEO juice, and authority attributed to these pages will be wasted. 404 Error pages that are not redirected will annoy users and make them leave your site immediately

The http error 403 is an error message that is occasionally displayed in the user’s web browser. The 403 error is one of the many hypertext transfer protocol responses that can appear when attempting to access a web page. If the ‘403 Forbidden’ message appears in your browser instead of the website you requested, it means that you are not authorized to access the specified URL.