Where \(\vec{E}\) refers to the electric field strength at the specific location, \(q\) refers to a "test" charge placed at that location, and \(\vec{F}\) the electrostatic force on that test charge.
This force is the net force due to all other charges in the universe causing electrostatic attraction or repulsion. We usually do not have to worry about charges that are too far away from the location being considered, as the force tends to be smaller when the charge is far away.
Just like for the gravitational field strength in Newtonian gravity, the electric field strength can be worked out "in principle" if you apply Coulomb's law. Let the location at which you want to find the electric field strength be the origin, and take the vector sum of the contributions to the electric field at that location from all the charges in the universe. The contribution to the field strength at the origin, from a point charge \(q\) that is at position \(\vec{r}\), is: