What it is
Operating cash flow starts from net income and adjusts out the accounting items that don't move cash (depreciation, working-capital changes, non-cash compensation) to leave the actual cash the operations threw off.
Why it can differ wildly from net income
A profitable company can run negative OCF if it's tying up cash in inventory and receivables. A money-losing company can run positive OCF if depreciation is large. The two numbers measure different things — both matter.
The first line of the cash flow statement
OCF is one of three sections on the cash flow statement, alongside Investing (buying equipment, acquisitions) and Financing (debt, dividends, buybacks).