These variables are specific to the problem, but the equation is a version of Coulomb's law.
It's an adaptation of Kq1q1/r^2(coulomb's) or Gm1m2/r^2(gravitational law), where R is the constant for some kind of attraction, n+e and n-e means one thing has a charge or some attractive property that is 2 higher than the other, and instead of r for radius, they used d for distance(same thing).