NThe number of civilizations in the Milky Way whose electromagnetic emissions are detectable right now, what the equation solves for.
R*The average rate of star formation in the galaxy, in new stars per year.
fpThe fraction of those stars that have planets.
neThe average number of planets, per star that has planets, that could actually support life.
flThe fraction of those habitable planets on which life actually develops at some point.
fiThe fraction of life-bearing planets on which intelligent life evolves.
fcThe fraction of intelligent civilizations that develop technology releasing detectable signals into space.
LThe length of time, in years, such a civilization keeps releasing those detectable signals.