I have an exam tomorrow of Engineer Economics and I don't understand what does "cash flow" mean. Can you please explain what does it mean and solve the exercise to see it applied. Thank you very much. (Picture attached)

1 (20 points). Given the following cash flows, year cash flow calculate the equivalent present worth, using 9% per year. $2000 $2300 $2600 $2900 $3200 $3500 $3800 $4100 10 $5200