Mean drift velocity is the average velocity of electrons as they move through a wire. Its symbol is v and its unit is ms-1. Charge carriers actually move slowly as free electrons repeatedly collide with the positive metal ions as they drift through the wire towards the positive terminal; the lights turn on quickly because all the free electrons in the wire start moving at once.
In this equation, I is the current, A is the cross-sectional area of the conductor in m2, e is the elementary charge (1.6 × 10−19), n is the number density of charge carriers (i.e. number of free electrons per m3), and v is the mean drift velocity of the charge carriers.
The number density determines the number of free electrons per m3 and therefore how good the electrical conductor is. Conductors have a very high number density (order of 1028m-3), insulators have a much lower value and semiconductors have a value between the two (order of 1017m-3).