There are several quantities out there within the world which are governed (at least for a short time period) by the equation,
Q = Q ekt
where Q0 is +ve and is the amount initially exist at t = 0 and k is a non-zero constant. If k is positive then the equation will rise up without bound and is called the exponential growth equation. Similarly, if k is -ve the equation will die down to zero & it is called the exponential decay equation.
Short term population growth is frequently modeled through the exponential growth equation & the decay of radioactive element is governed the exponential decay equation.