Simple if a market opens the people in that region become active in buying and selling so you have more volume/activity and the inverse when it closes. The most active periods would be when two markets of large groups of people are open at the same time, such as European and U.S. sessions overlapping each other or the Australian/Asian. The activity of markets around the globe is different because people got to sleep!
Comments
on the asset you are interested in and in time you will learn
which hours that work out the best for you.