What is Internal rate of return?
The discount rate at which a set of cashflows exactly breaks even, so their net present value is zero.
It exists only where money goes out and later comes back. A series of pure inflows or pure outflows has no break-even rate at all, and a series that changes sign more than once can have several.
Not to be confused with CAGR
CAGR describes one lump sum growing from a start value to an end value. IRR handles many cashflows at different times, which is what you need the moment money goes in more than once.