Scylla is described in Chapter 12 as a monster that has six heads. Each of these heads has three rows of teeth. Each head is set on a long neck. The creature sits inside her den and reaches out either her long necks to catch any sort of prey that comes close enough. This is what happens to the six men of Odysseus' crew.
As the ship avoids Charybdis, it inevitably comes too close to Scylla's lair. She reaches out with her six necks and grabs a man in each mouth. She then pulls them high up into the air and onto the rocks outside her lair. Having done that, she proceeds to devour them, even as they scream for help.
Here is a quote showing this:
...Scylla pounced down suddenly upon us and snatched up my six best men. I was looking at once after both ship and men, and in a moment I saw their hands and feet ever so high above me, struggling in the air as Scylla was carrying them off, and I heard them call out my name in one last despairing cry. ... Scylla land(ed) these panting creatures on her rock and munch(ed) them up at the mouth of her den, while they screamed and stretched out their hands to me in their mortal agony.