very well put not being locked to a specific carrier is very important
you can swap sim cards at will as long as you keep phone prepaid and dont committ to any lenghty contracts this is especialy important for travelers from what i understand if you sign a 1 year contract with a major cell phone provider and successfully complete your end of the contract the provider must unlock it free of charge at the end of your agreement
Most Cellphone providers lock their phones so it can only be used with the specific provider to ensure you don't switch to a different provider. Unlocked phone means it can be used with any provider. You can insert a sim card and start using the phone (very convenient if you travel outside of the country).
After your contract with the provider expires you can ask the carrier to unlocked your phone.