Hi, pippy
The worldwide forex market is available to participants, large and small, 24 hours per day from Sunday afternoon until Friday afternoon. But, not all participants are active around the clock.
Many large participants in the forex market — corporations, hedge funds, commercial banks (below the interbank tier) — operate on “normal” business hours, 8am-5pm local time, or something similar. When they are open for business, their impact on forex volume in their local markets is massive. When they are closed for business, forex volume in their local markets declines sharply.
This rise and fall in local forex volume is referred to as the “opening” and “closing” of the forex market in that country or region.
It may be that the forex volume generated in the United States is ten times greater at 11am New York time than it is at 11pm. We tend to refer to this ebb and flow of forex volume as the forex market being either open or closed, although really it’s that huge number of large market participants who are either open or closed for business.
Regarding commercial banks, the largest banks, which constitute what we call the interbank network, have trading desks which are open in many countries, 24 hours per day. Because of them, you and I can trade anytime.
But, because their largest customers are not open around the clock, the forex volume transacted by these giant banks tends to follow the sun.
That is, Deutsche Bank, for example, will do much more business in Asia during Asian daytime than at other times of day. Then, when evening comes in Asia, and Asian trading volume declines sharply, morning in Europe brings a surge in European trading volume.
Later, as morning comes to New York, a second surge of European volume reinforces the “opening” of the U.S. market, and the strongest market of the 24-hour day develops for a period of several hours.
I hope that helps to answer your question.
Clint
p.s. - Forex factoid: on the subject of the interbank network, altogether worldwide there are about 100 big banks in this group, but fully 60% of daily worldwide forex volume is transacted by the largest six of these banks.