XDA - 'unlocked'

  russ-rich 19:44 03 Jul 04

I have an oldish XDA 1 which I intend to put on ebay but I see other people when selling an item such as this saying it is 'unlocked'. What does this mean and is it good or bad to be unlocked?
I used it previously via pay as you go from O2 but it now has no sim card - I have transferred the number to another phone

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