The opening poem lays out the essential fact -- no human being can greatly affect the earth. Human lives are short, compared with the earth's history. And nobody can leave a permanent mark on that history -- eventually the winds and waves of time smooth out everything. (Consider an old gravestone -- usually it is difficult to read because the wind has gradually smoothed the letters, whether they were etched or raised.)
In light of this, Qoheleth wonders what should be the purpose of life, seeing how no human achievement can last forever. He considers some possibilities -- to pursue wealth, power, wisdom, justice -- but finds them all problematic. None can be perfectly achieved; none can prevent death.
So, Qoheleth argues that the best humans can do in life is "to eat, drink and find enjoyment in their toil." He expands on this in the best known part of the book, a poem that was set to music in the 1960s: "To everything there is a season." If one can figure out what season it is, one can do and say the right things. This is the best life one can live -- to find joy and fulfillment in one's day-to-day life.
Still, in this meditation, Qoheleth is not afraid to ask tough questions and challenge some conventional wisdom. So Ecclesiastes is a book that can be depressing, confusing, and provocative to the reader. Underneath it, though, is a persistent faith that human life is not meaningless and the human experience is not meant to be one only of suffering -- the faith that is explicitly expressed in Genesis 1 that God created people and called them good.