Also called a status code, an HTTP code provides an indication as to the result of a request.
These status codes are classified according to the type of message they return.
Among the most common are:
200 OK: this code indicates that everything was successful;
301 Moved permanently and 302 Found to signify that the URL has been modified, respectively, permanently or temporarily;
401 Unauthorized for unauthorized access;
403 Forbidden when the client does not have access rights;
404 Not found: by far the most common answer.
It occurs when the resource was not found by the server.
This happens for example following the update of a plugin or in the event of modification of a URL;
500 Interval server error and 503 Service unavailable to stipulate a server error.

1xx – Informational
This class of status code indicates a provisional response, consisting only of the Status-Line and optional headers, and is terminated by an empty line
100 – Continue
101 – Switching Protocols
102 – Processing

2xx – Successful
This class of status code indicates that the client's request was successfully received, understood, and accepted.
200 – OK
201 – Created
202 – Accepted
203 – Non-Authoritative Information
204 – No Content
205 – Reset Content
206 – Partial Content
207 – Multi-Status

3xx – Redirection
This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request.
300 – Multiple Choices
301 – Moved Permanently
302 – Found
303 – See Other
304 – Not Modified
305 – Use Proxy
307 – Temporary Redirect

4xx – Client Error
The 4xx class of status code is intended for cases in which the client seems to have erred.
400 – Bad Request
401 – Unauthorised
402 – Payment Required
403 – Forbidden
404 – Not Found
405 – Method Not Allowed
406 – Not Acceptable
407 – Proxy Authentication Required
408 – Request Timeout
409 – Conflict
410 – Gone
411 – Length Required
412 – Precondition Failed
413 – Request Entity Too Large
414 – Request URI Too Long
415 – Unsupported Media Type
416 – Requested Range Not Satisfiable
417 – Expectation Failed
422 – Unprocessable Entity
423 – Locked
424 – Failed Dependency
425 – Unordered Collection
426 – Upgrade Required

5xx – Server Error
Response status codes beginning with the digit “5” indicate cases in which the server is aware that it has erred or is incapable of performing the request.
500 – Internal Server Error
501 – Not Implemented
502 – Bad Gateway
503 – Service Unavailable
504 – Gateway Timeout
505 – HTTP Version Not Supported
506 – Variant Also Negotiates
507 – Insufficient Storage
510 – Not Extended

A 404 error indicates that the url is wrong or that the page never existed.
Either way, neither the Internet user nor the search engine can access the content.
However, too many 404 errors can impact the visibility of a site in the SERPs.
Search engines like Google do indeed prioritize user experience (UX).
An inaccessible page obviously does not go in this direction.
It is therefore quite logical that the SEO rating assigned to a site with too many 404 errors is downgraded.
However, 404 errors are not inevitable, their detection is easily done with the appropriate tools and plugins.
Once a 404 is highlighted, there are a number of ways to fix it, including a 301 redirect or changing the htaccess.
To spot any 404 standard errors on your site and measure the impact on your SEO, contact the SEO.fr team to get your diagnosis.