The bridge is the part of the atty that makes contact with the filler in a cartridge. It is usually covered with some wire mesh and it usually contains some silica wick to transfer the
juice to the coil. A bridge-less atty is one that was never intended to be used with a cartridge, so it was made without a bridge or silica wick installed. Also, for attys that were indeed made intended for use with cartridges, many people will remove the wick and bridge for a better dripping experience. --- If you are not using a cartridge, the bridge and silica wick serve no purpose.