An HTTP error is a code returned by the web server when a user wants to access a web page.
When this code is different from 200 (this means that the page is valid) or from 300 to 399 (this means a redirection), we can consider that the code is an error.
Among these codes, we can find:
You will find the complete list of HTTP codes on this article of Wikipedia.
Fixing these issues needs advanced technical knowledges. Contact a web developer to fix these issues.
These issues can come from an issue in the code, a server failure or an issue of a plugin of your website.