Camped on a cell: The MS (ME if there is no SIM) has completed the cell selection/reselection process and has chosen a cell from which it plans to receive all available services. Note that the services may be limited, and that the PLMN may not be aware of the existence of the MS (ME) within the chosen cell.
When camped on a cell the MS regularly looks to see if there is a better cell in terms of a cell re‑selection criterion (by using C1/C2 parameter), and if there is, the better cell is selected. Also if one of the other criteria changes, (e.g., the current serving cell becomes barred), or there is a downlink signalling failure, a new cell is selected. (This is called cell reselection).
Plz refer to GSM 03.22 for more information.