Most micrometers can read to one thousandth of an inch (0.001").
Vernier micrometers can read to one ten-thousandth of an inch (0.0001").
Consider the micrometer reading shown below:
If this was a 0 - 1" micrometer it would read somewhere between 0.255" and 0.256".
(250 on the Sleeve, with more than 5, but less than 6 on the Thimble).
Usually this would be close enough however some times we need to be more accurate.
Vernier micrometers have lines marked on the sleeve that let you read ten-thousandths of an inch. Each line is equal to .0001".
To show all the vernier markings I have "flattened" the sleeve of the micrometer. A real micrometer looks a little different and I encourage you to ask for a vernier micrometer so you can experiment with the real tool.
In the figure shown below, I have the same reading as before, only now we are using the vernier micrometer.
The above micrometer reads 250 on the Sleeve and 5 on the Thimble = 0.255. Now look for the vernier mark that lines up closest to any of the lines on the Thimble. In this case it is the vernier marked 8. The micrometer shown above reads 0.2558
It is easy to read what the micrometer is telling you. It is VERY difficult to accurately measure to .0001". This is due to the delicate feel that you must have to make this accurate of a measurement. Before you believe what your hands and micrometer are telling you, you must make several measurements to ensure you get consistent results.
You will quickly improve if you practice with a real micrometer.