The Drake equation was invented by astronomer and astrophysicist Dr. Frank Drake, to estimate the probability of life in our galaxy that might be expected to communicate with us.
“How can we estimate the number of technological civilizations that might exist among the stars? While working as a radio astronomer at the National Radio Astronomy Observatory in Green Bank, West Virginia, Dr. Frank Drake (currently on the Board of the SETI Institute) conceived an approach to bound the terms involved in estimating the number of technological civilizations that may exist in our galaxy. The Drake Equation, as it has become known, was first presented by Drake in 1961 and identifies specific factors thought to play a role in the development of such civilizations. Although there is no unique solution to this equation, it is a generally accepted tool used by the scientific community to examine these factors.”
Wikipedia has a more robust discussion of the equation here:
Some of the factors are well known, such as the average rate of star formation in the Milky Way Galaxy. Some factors began as vague guesses, such as the number of planets around each star, but are becoming more concrete knowable figures. Most of the other factors are actually educated guesses and may continue to be until we discover extraterrestrial life.
The Drake Equation exists mostly as a thought experiment, or as a framework to begin building a Search for Extraterrestrial Life (SETI), so it should be now surprise that Dr. Drake currently works for SETI.
Since most of the factors in the equation are estimates or guesses, the results differ. But generally the result demonstrates that the number of advanced civilizations that might be capable and willing to communicate to be in the order of millions. Or be as low as one.