what is unlocked phone?

[Mustasam]

please any one tell me what is difference between unlocked phone and other phone??? i want to buy s9 new phone. when i cheaked imformation from internet. there is too much type. please tell me which phone is best. unlocked or other??? and what mean by unlocked

[aivansison]

unlocked means the phone is openline. you can use any sim card on it.