in Coulomb's equation
F = k q1 q2/r^2
k is a constant = 1/ 4 pi epsilon0
epsilon0 is a constant of nature called the permittivity of free space; which is a constant of nature
the value of k is 9x10^9 in MKS units; the value of epsilon0 is 8.85x10^-12 F/m where F stands for Farads
When 2 charges exert a force on each other the force is proportional to Qq/r^2 k is just a proportionality constant to make them equal. So you get F = kQq/r^2 k is called coulombs constant.
We can also set k equal to 1/(4*pi*epsilon naught) So we can use either one. It may seem like using the epsilon naught term is just a weirder messier way of going about it but it's much nicer when you start doing other things like figuring out electric fields.
epsilon naught is for 2 charges in a vacuum, if they were in some medium that medium would have its own epsilon factor.