The statement is pointing out the continual nature of the alternation of generations life cycle. The acorn is a fruit, a dry one which contains a seed, product of an ovule, made by a female gametophyte and pollinated by pollen from a male gametophyte. These gametophytes are dependent on an oak tree, a sporophyte. The acorn makes the oak tree, which repeats these steps to produce more acorns.
Work Step by Step
Review the alternation of generations life cycle and describe it in a few sentences starting with the fruit (the acorn). The main point is that you could start the life cycle anywhere, not just with the sporophyte.