Internal rate of return (IRR) is the annualized return that ties together every dollar a property moves: your initial investment, the cash flow each year, and the lump sum when you sell or refinance. Formally, it is the discount rate that makes the present value of all those cash flows net to zero. In plain terms, it answers 'what yearly rate did my money actually earn, given when each dollar arrived?'
IRR's strength is that it respects timing. A dollar collected in year one is worth more than the same dollar in year ten, and IRR builds that in, which makes it the standard yardstick for comparing a rental against other multi-year investments. Its weakness is that it assumes you can reinvest interim cash flows at the same rate and can behave oddly with unusual cash-flow patterns, so read it alongside the equity multiple, which ignores timing but shows total dollars returned.