Investing cash flow (or CFI) is usually the 2nd portion of the cash flow statement and it measures the money that the firm use for its investment activities. The most important no. here is the Capex no. Now "Capex" may sound like a new brand of designer clothes but it's not. But maybe someone can start one hehe. Anyway, Capex is the short form for Capital Expenditure. This is what the firm needs to invest in (usually in new equipment, new technology or new offices etc) in order to stay competitive in its business.
This no. however is not labelled as "Capex" in the cash flow statement but usually goes by some obsure label like "Acquisition of New Property, Plant and Equipment". Why is this so? One good reason that I can think of is because auditors love to make life difficult for a lot of people and sell-side analysts, investor relations managers get to keep their jobs by having to explain what "Acquisition of New Property, Plant and Equipment" really mean.
Other no.s that are quite boring that goes into investing cash flow as well will include:
1) Sale of Property, Plant and Equipment (opposite of capex)
2) Purchase and Sale of Investment Securities
3) Acquisition of new subsidiaries or associated companies etc
One need not worry too much about those no.s unless they have a few more digits than the rest of the no.s in the cashflow statement. Meaning they are super big or super negative and it pays to know why that is so.
See also Cash flow statement