Since you are asking in PCB Design, I'm sure you mean PCB thieving. Thieving is copper added to open areas of a PCB to improve the 'copper balance'. By making the amount of copper coverage approximately even across the board it makes the board easier to manufacture; avoiding over- or under-etching of the copper features.
Thieving usually appears as an array of dots or diamonds added to what would otherwise be an open area on the board. If the thieving is added by the PCB fabricator, these bits of copper will be unattached to any net on the board, which may not be the best idea from an EMC perspective. So you may want to add copper fill yourself to open areas, attached to a ground net.
If there are areas in which you specifically do not want thieving to be added, you must designate this on your fabrication drawing and notes. You might want to prohibit thieving if it would interfere with controlled impedance elements on your board, or if you simply want a smooth area to place a label, or if it would violate creepage or clearance rules.
