It is where all the rules allowing access are stored. For example, in there you can place country or specific IP (or a range of) numbers to block access to.
For example, most servers have a black list of spam and hacking IP addresses.
My guess is that your IP number (which you may share with many other internet users) has somehow gotten blocked. Some browsers have built in VPNs (cheap, but all you need in this case), where you can change the country you appear in, and to do this they change the IP number you appear to be using.
Also, doing a forced bypass of your browser cache might be a good idea. In Chrome this is Ctrl + F5 when on the page you want.