See the exam page for more details. Today's lecture discusses half of the field of economic geology, in particular energy resources on the Earth. You need to keep in mind that things may well change, and change radically, in the future. The fraction of resources of a given energy source which is known to exist. This would, for example, be the amount of oil we know exists in the world.
Reserves are by definition smaller than resources.
The fraction of reserves of a given energy source which can be extracted and used for a price which is affordable in a given economic environment. When the supply of oil, say, is high, there is no incentive to explore for oil in difficult areas. However, in the future prices may rise, which would make it economically feasible for oil companies to explore and exploit areas which are now just too expensive. Oil found in those areas would then be part of our economic reserves of oil. By definition, economic reserves are less than reserves, which are in turn less than resources.