In the dye industry, a mordant is an additive that helps bind a dye to a fabric.  In other words, it helps make the dye colorfast.  Most mordants consist of ions or charged metal complexes.  Congo red does not require a mordant to remain colorfast to cellulose fibers in fabrics.  It is an azo dye, meaning that it contains nitrogen nitrogen double bonds.  But congo red also contains two sodium salts in the form of sulfonic acid salts.  This is where the hydrogen in sulfonic acid (SO3H) has been replaced with sodium (SO3Na).  This means that in water or alcohol solvent, congo red is a charged, ionic species.  This allows the dye to adhere to the fibers without the use of a mordant.  But congo red is not used anymore in the dye industry due to potential toxicity issues.