At the risk of repeating some of what Zorro said - if you clip the clamp to your metal table and then lay your workpiece on it, the work piece and the table should form a circuit so the current can flow from the rod to your workpiece to the table and finally back to the clamp. Where this doesn't work is the situation when rust or some other coating prevents a good contact to be made between the workpiece and the table - remember, the contact area betweent the workpiece and the table have to carry the full welding current. Sometimes you will see sparking at the junction of the two and will get pitting on the surfaces caused by the arcing. If this happens then you can put the clamp directly on the workpiece or make a better contact with the table.
Putting the clamp on the workpiece is the best but often the weight of the clamp and cable make aligning pieces difficult. The welding circuit is not referenced to ground so current should not be trying to flow to ground. You should be able to touch the stinger and a ground and not get a shock (I do NOT recommend you try this in case there is some poor insulation) but if you touch the bare tip of the rod and the clamp you will get a tingle from the
50-80 volts. Good electrical practice says to keep your gloves and boots dry when handling the clamp or the stinger when the welder is turned on since they will provide insulation for your body.Zorro brings up a good point to repeat since it can cost some needless expense if forgotten. If you are welding on a car put the clamp as close to the weld area as possible so the current through the car body can go directly to the clamp with the absolute shortest path. This greatly reduces the chance of currents getting into sensitive electronics. Some weldors disconnect the battery also. If you are welding on something like a wheel hub make sure the clamp is on the hub not the car frame so the current does not have to pass through a bearing to get to the clamp. The current flowing through the bearing rollers/balls will likely arc and cause pitting.
In other words no matter what you are welding (including on your table) think about how the current is going to flow from the rod back to the clamp!
billh