The dominant line is usually capex. Acquisitions sit here too, which is why an acquisitive company's investing outflow tells you far more about its strategy than its income statement does.
Purchases and sales of marketable-securities also run through this section and can make the total lurch around for reasons that have nothing to do with the business. Read the components, not the subtotal.
Example: Northwind Tools shows negative $92M: $75M of capex, $31M for a small distributor acquisition, less $14M from selling an old plant.
Related: capex, cash-flow-statement, free-cash-flow, marketable-securities, capital-allocation