what is a statement of cash flow and is it an optional financial statement?

Cash flow is similar to the income statement in that it records a
company's performance over a specified period of time, usually over
the quarter or year. The difference between the two is that the income
statement also takes into account some non-cash accounting items such
as depreciation. The cash-flow statement strips away all of this and
tells you how much actual money the company has generated. Cash flow
shows us how the company has performed in managing inflows and
outflows of cash. It provides a sharper picture of the company's
ability to pay bills, creditors, and finance growth.