What does this equation mean?

where the first right-hand term carries the probability density along a deterministic drift $\mu(x,t)$ and the second spreads it by diffusion at rate D(x,t) [ 3 ] . Andrei Kolmogorov arrived at the identical equation independently in 1931, from pure probability theory, as the forward equation governing how the probability distribution of any continuous-time Markov process evolves [ 3 ] . Kolmogorov’s work also produced a companion equation, run backward in time from a fixed outcome rather than forward from a fixed start, that turns out to be the more convenient tool for asking what a process is heading toward rather than where it began — a distinction that mattered when the same mathematics was pointed at a population instead of a particle. That third, unrelated derivation is what turned Einstein’s equation from a fact about pollen grains into a piece of mathematics indifferent to what its variable represents — a state of affairs population genetics was about to test.