The flanking line is a line of cumulus towers that are being drawn into the supercell's main updraft, instead of forming a line squall or other linear storm system. By being drawn in, it strengthens the storm's main updraft as the towers combine into a single tower. This is what enables supercells to have the continuous, long-term updraft that makes them so powerful.

The following picture depicts the flanking line on a supercell: flanking line