What does each factor in the Drake equation represent?

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artist's conception of Kepler-186f — extraterrestrial intelligence An artist's conception depicts Kepler-186f, the first Earth-sized extrasolar planet found in its star's habitable zone.

The Drake equation is a probabilistic formula used to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way Galaxy. It is expressed as $N = R_{*} f_{p} n_{e} f_{l} f_{i} f_{c} L$ , where each factor represents the following:1

$R_{*}$ : The mean rate of star formation in the Galaxy.1
$f_{p}$ : The fraction of stars that possess planetary systems.1
$n_{e}$ : The average number of planets in those systems that are ecologically suitable for the origin of life.1
$f_{l}$ : The fraction of those suitable planets on which life actually develops.1
$f_{i}$ : The fraction of planets with life where intelligent life forms evolve.1
$f_{c}$ : The fraction of worlds with intelligent life that develop high technology capable of interstellar radio communication.1
$L$ : The average lifetime of such advanced civilizations.1

Because these variables are difficult to quantify, the equation serves primarily as a framework for discussing the uncertainties surrounding the existence of extraterrestrial life.1