Not an accountant, but
IIRC Full cost is the annual cost of everything to do with that airframe including all the support staff, buidlings, etc divided by the total number of hours flown in a year. i.e. all direct and indirect costs
The marginal cost is the cost of one more hour in the air ignoring all the indirect (fixed) costs, i.e. fuel and maintenance. Full cost per hour will vary with the number of hours flown, marginal will not.