The Drake equation is an effort to calculate the probability of the number of advanced civilizations in the galaxy. Each variable in the equation expresses a factor that is believed to affect the outcome regarding the development and longevity of alien life, regardless of its location in the universe.
The Drake equation seeks to determine the number of advanced civilizations, with "advanced" meaning "capable of radio communication," by starting with the rate of star formation in the Milky Way multiplied by the fraction of those stars that have planets. This product is multiplied by the number of planets that have conditions suitable for life, multiplied by the number that actually do support life. These products are then multiplied by the fraction of alien life forms that have evolved intelligence, and then multiplied by the fraction of intelligent populations that have developed radio technology. Finally, the product is multiplied by the length of time the civilization's radio technology released signals into space. The final product is the number of advanced civilizations that are theoretically detectable from Earth.