I found this command lines on the net:find . -type f -exec chmod 644 {} +find . -type d -exec chmod 755 {} +I'm not sure of what they do when executed... In theory I guess they go search of all the files and convert them to permissions 644 and the second line search for all the folders and convert them to 755 but I don't thing that I did anywhing once I typed enter. Also, I needed these line because I wanted to to set these permissions for my wordpress configuration, but I accidentally typed these commands in the / directory and not in /var/www/html/wordpress.... Can I stay calm or I did something wrong and my server will have problems? By now, it seems to work normally...Still one thing: can you tell me the best and fastest way to switch the permissions of all the files inside /var/www/html/wordpress to 644 and all the folders inside /var/www/html/wordpress to 755?--UPDATE--I checked the history of terminal and it seems that I was on /root/ home when I executed these lines, so it's a great news! 
chmod - What does this command do?
permissions;find;chmod;wordpress
It's easy to recover from an error like that with a RHEL-based distribution. But with Debian, at this point, the easiest thing to do is to reinstall Debian. Next time you have to write:find /var/www/html/wordpress -type f -exec chmod 644 {} +find /var/www/html/wordpress -type d -exec chmod 755 {} +