cron job is a scheduled process that runs on a server every so often and does what you tell it to do.
http://en.wikipedia.org/wiki/Cron
virtually every single hosting service offer some sort of cron capability. either you set it up yourself or through whatever facility hosting company provides you or you ask them to set it up for you.
if you'll read url above you'll see that hourly cron would look like
0 * * * * chmod 755 -R /path/to/your/files