The BBC is legally obliged to prevent overseas viewers accessing most of its content. (Either the BBC doesn't hold the international broadcasting rights for that content or it does hold the rights but wants to sell them to broadcasters in other countries).
So the BBC's servers examine the IP address of every computer that's seeking to receive content from BBC iPlayer and block access to any that aren't seen as being in the UK.
Using a VPN ( = 'virtual personal network')
might help someone get around the block but only if the servers for that network are based in the UK. (Most aren't).
To be sure of getting around it, a user needs to use a service that routes their traffic via an anonymous UK-based proxy server. Even then, there can be no guarantee that it will always work, since the BBC (and other services that seek to restrict access to UK users) can often detect that a proxy server is in use, simply because of the volume of traffic that passes through it. However this is the most popular software for getting around the block and generally seems to work:
http://expat-shield.en.softonic.com/