I am shopping for disc brakes and looking at cable-actuated ones. What would "pre-bled" mean in that context?
It means the brake caliper/housing/lever are supplied connected and ready to fit to your bike. Additionally, the brake fluid is installed, and all the air bubbles have been removed.
So if you install without shortening the hose or running the hose internally through your frame, then its a mess-free task.
If you need to shorten a hose or run inside your frame for any distance, then there's a chance of introducing air bubbles inside the line which decreases brake effectiveness, and needs bleeding to get the air out. Not a big deal, but its a process with steps plus you'll need some additional fluid and tools.
Your cable-actuated hydraulics are just hydraulic brakes with some additional cable between the lever and the cylinder. So the pre-bled bit is the hydraulic part, however small the hose and fluid section might be, it comes filled with fluid and without bubbles inside.