Auto Key framing in Blender eliminates the task of pressing the "I" key o get the Insert popup screen where you click on a particular option to insert a key frame for the selected object.
The steps to do auto key framing are listed below:
1. Let us assume that you want to animate the translate operation of an object, by moving it from one point to another in a sequential manner. Select the object you want to animate.
2. Position the object at the initial position from where you want to start the animation.
3. Move the cursor(green line) in the Timeline editor to frame number one.
4. Position the 3D cursor in the 3D window. Press I key, in the popup menu, click on location. A key frame will be inserted at frame number one which is indicated by an yellow line.
5. Press the auto key frame button(red button) in the header of the Timeline window. Activation of the auto key frame is indicated by the background color of the auto key frame button changing to dark grey color.
6. Now move the cursor to say frame number 20, then move the object to the next location and press LMB to confirm the location. You will see a key frame is inserted at frame 20, indicated by an yellow line.
7. Repeat step 6 for each new location of the object.
8. When you want to stop auto key framing, press on auto key frame button to de activate this function.
9. Please note that to start auto key framing the first key frame must be set manually.