You can hover over each file in either of those lists to see the reason for them being on that list.
For the skipped files it is either because the file size is 0 Bytes (which means it contains no code that could be scanned) or the file type is on the list of file extensions to be skipped (such as CSS, language files, images, fonts, and other binary files that cannot be executed by CGI engines like PHP on the server).
As for the Read/Scan Errors, these are more varied and you should look at each one to see if you can determine why the scan process could not read those files. Sometimes it is cause by improper permissions on the file other restrictions on the server. You could try increasing the memory_limit value set by the php.ini file on your server to make sure your PHP Processes have access to enough memory to read copy and scan all the file’s content (this may also dramatically increase the overall speed of the scan in some cases). Ultimately, the answers for what is causing any error on your site should be found in the error_log files on your server (ask your hosting provider where you can view these logs).