Drawboring is a traditional technique for tightening and reinforcing a mortice and tenon joint.
A hole is bored transversely through the mortice and another (fractionally offset) through the tenon. Drawbore pins are then inserted from either side and twisted so that their slightly eccentric grind pulls the joint up tight.
The drawbore pins are removed and replaced with a very dry hardwood peg.
Once you become aware of them you will find drawbored mortice and tenon joints are almost ubiquitous in early furniture and common in better quality later work. The same technique is still used extensively in timber frame building construction.
The ⅜" are generally better for typical furniture scale work, whereas the ½" are suitable for the larger joinery of boatbuilding and architectural work.
Delivered as a pair.