This passage, one of the most frequently quoted from Ecclesiastes, means that there are events in life which no one can avoid. There is a time to be born, and also a time to die. The time of one's death is not of one's choosing, yet it comes. Similarly, when crops are ripe, then it is time to harvest; again a time that "arrives," not a time that one chooses. Similarly there are times of happiness and sadness; times when one experiences gain and others when one experiences loss. "rending and sowing" is a metaphor for dying (one would rend his clothes when a loved one died; sowing implies birth.)
In essence, the passage means that there are many events in life which happen simply because it is time for them to happen, and humans have no control. It is perhaps best expressed by another verse in Ecclesiastes:
Vanity of vanities, says the Preacher, vanity of vanities; all is vanity.