In agroforestry, when lines of trees are planted room is left for compatible plants to be grown. These will improve the soil, providing nutrients for the trees, especially valuable nitrogen. You can regard the lines of plants between the lines of trees as 'alleys' and this is what alley cropping means- the cultivation of food, forage or specialty crops between rows of trees.
It is a larger version of intercropping, which is a similar planting of compatible plants between trees, but conducted over a longer time scale. Alley cropping provides profitable opportunities for farmers, not just for their crops, neatly in rows, but for growers of hardwood timber, nuts and Christmas trees.