what is Resistor?what is its function?

Basically, any material substance has resistance, i.e. it [resists] the flow of current in a circuit. A resistor is just an electrical component, ususally made from carbon films or some alloy wires, that is introduced in a circuit for the fixing the current or voltage in the curcuit at the designed values.

Resistors, when put in different forms, can produce various functions. When current passes through a resistor, heat is produced. This phenomenon (heating effect) is used in the heating element in heaters to warm our houses in winter. When used in a light bulb, the tungsten filament inside the bulb (which is, in fact, a resistance wire) produced light when current is passing through.

A resistor is a two-terminal electrical or electronic component that opposes an electric current by producing a voltage drop between its terminals in proportion to the current, that is, in accordance with Ohm's law: V = IR. The electrical resistance R is equal to the voltage drop V across the resistor divided by the current I through the resistor. Resistors are used as part of electrical networks and electronic circuits.
Function?
I KNOW YOU WANT TO QUESTION (Function) BUT I DON'T KNOW YOU WANT TO TALK ABOUT

Function (biology), explaining why a feature survived selection
Function (mathematics), an abstract entity that associates an input to a corresponding output according to some rule
Function (engineering), related to the utility/goal of a property
Function (computer science), or subroutine, a portion of code within a larger program, performs a specific task
Function object, or functor or functionoid, a concept of object-oriented programming
Grammatical function, in language studies, the purpose of a word or phrase in a sentence
Diatonic function describes a music term
A formal event such as a party or meeting