What does "bonding" mean with respect to wiring in an outbuilding?
Bonding in the context you have provided means to connect or otherwise attach one item to another. Your grounding electrode system can be and frequently is an eight-foot copper clad rod embedded in the ground. Depending on the composition of your earth, it may be necessary to increase the length and depth. Consult your local codes regarding earth ground requirements.

The other part of the bonding is the segment of copper wire attached to the aforementioned ground rod. That is frequently a simple clamp available at big-box hardware stores and is called a grounding clamp. Imagine that. The ground wire also should meet local codes regarding wire gauge, often number six gauge (or at least in my area it is) and is fastened to the ground rod at one end and secured to the enclosure/panel at the other. The ground at the panel shares the connection with the ground lug for the breakers as well.