This is an estimation technique from “traditional sciences”, named after italian physicist Enrico Fermi.
Roughly: this technique is used to get an estimate of order of magnitude of a problem, based on known lower and upper bounds. For example, if you were to ask me, “How long would it take humans to build a flying personal car, like in sc-fi books from the 50’s?”, I could use fermi estimation to give a ballpark estimate.
We know it will take less time than it took humans to realize + build spaceships, but more time than it took us to invent the smartphone.
So, if it took Nasa nearly 10 years to build The Enterprise, but it took apple two and a half years to ship the iPhone. So I’d guess, assuming all the underlying technology is already invented, that it would take 2.5-10 years to build a flying personal car.