Question

What is resistance ? what is the function of resistance

Answer 1

The electrical resistance of an electrical conductor is a measure of the difficulty to pass an electric current through that conductor. The inverse quantity is electrical conductance, and is the ease with which an electric current passes. 
The main function of resistors in a circuit is to control the flow of current to other components. Take an LED (light) for example. If too much current flows through an LED it is destroyed. So a resistoris used to limit the current.

Answer 2

It is a device which resist the the current passing through the conducter.Because access current can destroy the electric appliances.