A pro forma is a projection: a spreadsheet view of what a property might earn and cost over a future period, from gross potential rent down through vacancy, operating expenses, NOI, debt service, and cash flow. Every listing brochure has one, and every serious buyer builds their own. The word to hold onto is "projection" — a pro forma is a set of assumptions wearing the costume of a financial statement.
The gap between a seller's pro forma and reality is where deals go wrong. Common tells: market rents applied to units that have never achieved them, a thin vacancy allowance, property taxes at the seller's current assessment rather than your post-sale reassessment, and maintenance numbers with no capital reserve. The antidote is to underwrite from actuals — trailing-twelve-month statements, the real rent roll, real tax and insurance quotes — and treat the pro forma as the hypothesis your due diligence has to confirm.