An archon toilet is the idea of using a mothership's vortex to trap the enemy's army, then to dump a few archons into the vortex. When the vortex finishes the archons will attack immediately, and because of their splash damage they will basically insta-kill the whole army when it's clumped together.
Of course this is all pre-1.3 patch

, now there is a small time period afterwards where all units are immune to damage, hopefully reducing the effectiveness.
edit: I believe this daily shows it quite nicely, it's somewhere near the end: