Many mobile phones when sold will only work on certain networks, to enable you phone to used on other networks and so choose which sim card you can use your mobile phone, requires you mobile phone to be unlocked.
This is not something anyone can do, only those with the right equipment and knowledge can do this, but the good news is that it is normally cheap process with some people charging as little as £5 to do this. Market stalls small shops tend to be everywhere and as well as selling mobile phone accessories, second hand phones and chargers, they will normally do an unlocking service for you.
With an abundance of second hand mobile phones everywhere, the charge to unlock your phone, can only represent what the service is worth to the customer and quite frankly if it were to be too expensive it would be easier for the consumer to source a mobile phone on the correct network anyway.
Most unlocking tends to be for Pay as you go phones, because most rental offers would give you phone free, but afetr that contract has expired you now have a phone you can only use on the network and those who now want to change to pay as you go would consider getting this phone unlocked t use with other sim cards.
Some of the big retailers also offer this service, but it will cost more as they would want their cut also as well as postage, collection etc.