80% of unlocking jobs are done on the spot in our Tampa location or the same day

What's an unlocked cell phone? And what is GSM

An unlocked cell phone can be used with any SIM card from any carrier. A locked cell phone or smartphone will only recognize a SIM card from a particular carrier.
If you purchased your GSM cell phone at a discounted price, it is more than likely that the carrier has "locked" your cell phone or smartphone so it will only work with their service. This is designed to prevent you from switching to another GSM carrier.

The term GSM means - Global System for Mobile Communications. It is the most popular standard for mobile phones in the world. GSM differs significantly from its predecessors in that both signaling and speech channels are Digital call quality.
GSM is the most widely system used worldwide….period

Once you  unlock your GSM phone, you're free to use your choice of SIM and service with the phone, potentially saving you substantial money and adding to the flexibility of your phone. If you have T-Mobile or AT&T and they try to sell you an international roaming package because you are going overseas, STOP, do not do it, it is much economical if you just get an unlocked phone and buy prepaid sim with minutes in the the country where you are going to.