What does a resistor do?
Electric flow is the motion of electrical charges through a material. Resistance is the physical obstruction of these moving charges.

A certain amount of energy is required to keep these charges in motion, and since the energy drop is proportional to the amount of charge kept in motion, this results in a voltage drop across the material since electromotive force (in volts) is energy (in joules) per charge (in coulombs).

Since it is a physical obstruction, it also restricts the rate at which charges can move across a given point per unit time. This results in a maximum current, since current (in amperes) is charges (in coulombs) per unit time (in seconds).

And as it turns out, if you apply more or less electromotive force across the same resistance, the current increases or decreases exactly linearly. This gives rise to Ohm's Law, which states that electromotive force is proportional to the product of current and resistance, that is, \$E = IR\$.