Meter in refers to the restriction of a hydraulic circuits inlet flow. They may only be used when utilizing opposing loads as these systems have no way of preventing a load from running away. These are used to regulate motor speed and ram speeds.
Meter out refers to the circuits restriction of the outlet flow. i.e. a hydraulic ram used for offloading heavy material such as a car lift. A meter out system would be used to regulate the flow evacuating the system as to prevent a run-away and maintain load control.