A 403 Forbidden is the HTTP response status that indicates that the request has been understood by the server, but it is denying the user access to the resources they are looking for. This usually happens when content is limited to registered users or administrators and restricted for all others.
A 403 is the HTTP response status code that indicates to the user that thy do not have permission to access a resource on the server. This may happen when content is limited to registered users or there is a precondition of some sort to accessing the data.
This sort of error may happen when you are trying to write an exact URL in your web browser, pointing to a file in the structure of a website, but that file is not accessible for common users, only for site administrators. This can be for security, privacy or functionality reasons. The error may also be the result of the fact that the owners of the server are improperly setting permissions (read, write, execute).
Whatever the reason behind it, the 403 forbidden error can only be fixed by the person who has direct access to the files on the server. As an internet user there is not much you can do about it, except ask the webmaster for permissions, which they may or may not give, depending on the nature of the resource you are trying to access.