Trees have already moved most of the food energy from the leaf tissues to storage in branch, trunk and root structures. The remainder is pretty much pure organic matter, the raw material for humus. It's a complex end product of plant and animal residue that has thoroughly decomposed. It lodges in the cracks and pore spaces between the tiny pieces of sand or clay that make up the bulk of soils. In sandy soils, the spaces are larger, making it hard to retain moisture.
Clay soils have the opposite problem.
Western soils are especially notoriously lacking in natural humus and undecomposed organic matter. Centuries of low rainfall and short growing seasons have produced little disposable greenery like that in tropical areas. Thus, to improve our clay or sandy bases, it's up to you to add the organic matter. Leaves, dried grass clippings and residue from hand or power raking are all candidates. Commercial composts, Canadian sphagnum peat moss and treated animal manures also can be used.