Rebuildable Dripping Atomizers (RDAs) do not have a holding tank for eLiquid. Instead of filling up your tank, you will drip a little bit of your eLiquid right onto the coil and start vaping. Dripping often results in richer flavors and a better throat hit.
Rebuildable Atomizers (RBAs) do have a holding tank for liquid so you don't have to drip.