In a recipe on Corned Beef and Cabbage, the instructions read "wedge" the head of cabbage before putting it in the slow cooker.
Another way to say this is to quarter the cabbage. The goal is to make "wedges" (triangles, when viewed from above) that are smaller than an entire head of cabbage (easier to cook and fit in your slow cooker) but still relatively intact.
To wedge a cabbage, slice it in half, and then slice the halves into either 2 or 4 pieces (depending on how large a wedge you want).
Image from Green Lite Bites