The knuckle is the assembly that pivots on the big ball at each end of the front axle. It has several components, including your front wheel bearings and spindle, the Birfield CV joint and stub axle, and locking hubs. The brake calipers mount to the knuckle, as do the relay rods for your front steering. Rebuilding the knuckles is a messy job but well within reach of a person with moderate mechanical ability. I did mine in my driveway over a weekend.
Anyway, if you see oil dripping from the bolts on the bottom of the knuckle, it means that the oil seal in your front axle has failed and gear oil from the diff is leaking out the end of the axle housing. You have to remove the whole knuckle to replace the seal, so that's as good a time as any to rebuild your front end. Might as well do tie rod ends and brake pads, too, while you're in there, haha. It's not something you have to do often, and your truck may not need it right away, but something to keep in mind as you work to make your truck a reliable go-anywhere vehicle.