Extension tubes are metal tubes with a rear-lens mount at one end and a camera-body mount at the other end. Extension tube set does not affect image quality as there is no optics inside
No electronic contact and auto focus cannot be performed. Exposure and focus have to be adjusted manually
Use of:
set your lens to maximum aperture;
set-up camera and lens to manual mode, switch off and detach the lens;
attach extension tube between the camera and lens;
put your camera on tripod, or better on rail system;
put object close to the lens and use lot of light;
increase ISO, decrease shutter speed;
Please note: when tubes attached, you need to do all manually.