Central banks play a key role in the currency markets because of their power over monetary policy. They have a direct influence over money supply, which in turn affects demand and price of the currency. Through the use of different policies, central banks can try to manipulate the markets so that they can keep their currency at specific levels. Some countries and their central banks try to peg their currency to that of another currency or basket of currencies (for example, China).
The central bank can participate in the forex market by buying and selling their currency at the spot market in order to keep it from changing too much. Another motivation for central banks is to keep the local currency at a specific price in order to make it more attractive for trade. If a country's currency appreciates too quickly, it could actually make it less appealing to importers.
Remember that many transactions have to use the local currency. Thus, if currency that is needed rises too quickly, it effectively makes goods more expensive, which in turn, hurts trade. To counter this, the central bank may intervene in the market by selling its currency and buying up other major currencies. This in effect, weakens the local currency so as to make it more appealing to foreign importers.
While the exact value of what percentage such central bank transactions take up isn't known, take note that because these are the banks of national governments, such interventions can have a much larger impact on the market than any single commercial bank.