Noise effect in resistor

It is the random noise generated as the current flows through the resistor, or look upon it as very small random changes in resistance.
Imagine hooking your 'scope onto your perfect 5v supply, you see a perfectly flat line at 5v, make a divider from two equal resistors across that 5v and hook your scope on the mid point, you'd expect to see a perfectly flat line at 2.5v, but you will see 'noise' on that 2.5v the ammount of noise will depend on the material the resistor is made of and is generally proportional to temperature. (Of course there will also be RF pickup and loads of other 'real world' stuff depending on your exact setup.)

I can't offer a good explanation of the cause, so I won't offer a load of waffle, maybe it's the infinte possible paths for an electron as it passes through the resistor...whatever...

Del

You gave a good answer.

The original post asked what is meant by noise effect in a resistor not please explain the physics in excruciating detail. Sometimes we engineers like to go in to painful detail before we know how much detail is desired or even understood.

You may find this helpful:

http://www.ecircuitcenter.com/Circuits/Noise/Noise_Analysis/res_noise.htm