Why is my SIM showing locked?

If you originally purchased your iPhone as part of a cellular carrier contract your SIM is locked to that carrier until that carrier releases it. If you bought the iPhone second hand the original owner may have sold you a locked phone.

Look at Settings->General->About, scroll down to Carrier Lock and see what your status is.

The phone is locked to the cellular company it was purchased from. This is done when purchasing phones through them until the phone is paid off. The phone will not be able to be used on any other carrier and can only be removed by the cell provider that locked it.

My SIM card is locked