6 ideas
Qoheleth argues that toil yields no lasting profit (yitron) because whatever a person builds or amasses must be left to a successor who may be a fool. Death cancels the ledger, so the value of labor cannot lie in what it accumulates.
The narrator, speaking as a king, runs a deliberate trial: he builds houses, plants vineyards, gathers servants, silver, singers and concubines, and denies himself no desire. When he reviews the results he finds it all hebel, vapor and chasing wind, showing that stacking pleasures does not add up to lasting satisfaction.
Every human action has an appointed time within a cycle of paired opposites: birth and death, planting and uprooting, tearing down and building, silence and speech. Wisdom here means reading which season you are in and acting to fit it, not applying one rule everywhere. Humans can sense that there is a larger order but cannot see it whole.
Qoheleth holds that wisdom beats folly the way light beats darkness, yet more knowledge brings more sorrow because it shows how much is crooked and cannot be straightened. The wise man also dies just like the fool and is forgotten just as fully, so wisdom brings clarity but no escape.
Since lasting profit is impossible, Qoheleth tells the reader to eat bread with joy, drink wine with a merry heart, and love one's companion within the brief days given. Enjoyment is framed as a portion handed out by God and received in the present, not a reward earned by control or accumulation.
A great king besieges a small city with few defenders, and a poor wise man inside saves it by his wisdom. Afterward no one remembers him, which shows that wisdom may be stronger than weapons yet the poor man's wisdom is despised and his words go unheard.