what is the role of qin dynasty in the history of china

Before Qin Dynasty, there were many states in China and Qin Dynasty was the first country to unite the whole country and found centralized country.
The country made the stardard weights and measures for the country. Currency was also united by the central government.
It was also the first country to connect the Great Wall in many states together.
These are all I know. I suggest you to look more detail information on Wikipedia and I am sure you can find more information.

thanks john for your answer

your welcome