This describes an exponential growth curve. Y is population size (perhaps cell number) and X is time. At X=0, Y equals START. Y increases geometrically with a doubling time equal to 0.6932/K.
Note: It is difficult to fit data to this equation with nonlinear regression, because a tiny change in the initial values will drastically alter the sum-of-squares. You may need to override the initial values provided by Prism.