Firstly, your parents need to sign up with an ISP who will enable ADSL on their phone line. (This is assuming that they don't have Virgin's Cable service in their area.) The ISP should provide them with a Wireless Broadband Router and probably a CD from which to run a wizard to set it all up.
Once set up, any computers connecting "wirelessly" will scan for Available Wireless Networks, you (parents) select the one whose SSID (network name) matches that of their router and click the option to "Connect". A prompt will appear to enter the encryption 'key' and should then be connected to the router. A profile is saved so that connection in future is automatic when the network is available.