An unlocked phone allows you to switch between wireless carriers. A locked phone can only be used with the carrier it is locked to. Unlocked devices are generally GSM & CDMA compatible and will work on most major carriers.
All unlocked phones require the use of SIM cards. Mobile Klinik devices do not include SIM cards. If you already have a compatible SIM, you can insert it into your new device for immediate activation. Otherwise, you will have to source one from your chosen carrier.