A white glass cannon.
Upon activation, Isaac will raise the cannon above his head, lowering his movement speed. On pressing a fire button, Isaac will fire a enormous tear in the chosen direction. Additionally, Isaac's red hearts are set to half a red heart. Soul hearts and black hearts are not affected, unless Isaac has no red hearts, in which case you will be reduced to half of whichever heart was first in your meter.
A "glass cannon" is a common term in RPG games for classes that deal a lot of damage, but have low health.