International Mobile Equipment Identity is like a serial number for devices that connects to cellular networks. This basically tells the carrier what type of phone is being used which might be important for SIM locking. The IMEI will also be able to identify compatibility with the network. This is good in case your phone is a unique one with the local market. Phones can have 2 IMEIs if they are a dual SIM? phone.
An IMEI does not identify if the device is locked to a particular carrier. Carrier locking will require that the customer to contact the carrier to unlock the phone. Me personally, I feel device locking should be considered as illegal, and people should just purchase their phone outright. I would only buy from the manufacturer which helps assure that the phone is not locked.