The whole concept is this. The electrostatic force is a force that acts from a distance, its magnitude inversely propotional to the square of the distance between the charges and directly propotional to the product of their charges. Now, as the force acts from a distance, there must be a way for a charge to know the presence of other charges, so we take it that every charge has around it a field of influence such that, when another charge comes in influence of the field, it is attracted to or repelled by it, depending on its nature of charge (positive or negative)
Field lines are imaginary lines drawn from the charge which have these 2 interesting properties which make them so useful
1. The number of field lines per unit area at a position tell us the intensity of the field.
2. Tangent to the line at any point gives the direction of the field.