Capacitance is the ability of an object or surface to store an electrical charge. It is simply a measure of the capacity of the electrical storage capability of the object.
Capacitance is analagous to the ability of a jar to hold a fluid.
Just as some jars have more capacity than others, some objects have more charge-holding ability than others. Like the jar, this ability is related in part to physical size. It may sound wierd, but the electrical capacity of an object also depends on what the object is near to, and how near it is. All objects obey this rule, and this principle has been known for a couple hundred years. This is called mutual capacitance, and this form of capacitance is much more important and usually more dominant than simple 'free-space' capacitance.