Discipline helps the trader overcome two types of emotions encountered when trading:  Fear and Greed.
Manage risk.  Trade better.
Pile of international currency bills.
Simply, discipline or self discipline is the effort and determination of a trader to adhere to:
     1.  A trading system
     2.  Specific money management rules he/she set for trading that system.

Group of people doing yoga exercise.
First, adhering to each and every buy, sell, and hold forex trading signal generated by a thoroughly tested trading system eliminates fear of establishing, maintaining or exiting a position in the forex market.  Further, abiding by a forex trading signal does away with greed that often causes foreign exchange trading pitfalls resulting from getting into a position early because of impulse to by or sell,  staying in a position too long hoping to accumulate more profits and eventually exiting it late, sometimes turning a good trade into a losing one.
Greed.  A viable trading system, money management, and discipline eliminate the pitfalls of greed in trading.
Financial data on a LCD screen.
Second, adhering to preset money management rules of a thoroughly tested trading system eliminates the greed to make a larger profit by putting too much cash (overtrading) in a single position that might become a losing position.  Adherence to money management rules also eliminates fear that can result in investing less money into a trade that might eventually be profitable.

Self discipline is an an ingredient of successful foreign exchange trading.  It is a valuable quality that can be developed and learned through currency demo trading and actual trading.
Broker working in front of computers on the exchange floor.
ForexThinker.com currency trading formula:

money management + viable trading system+ self discipline=
Successful Currency Trading


                     Q:  What can
                      be a costly
                      mistake for
                      a trader?

Ans:  Placing an incorrect

View at the Chicago River at night.  Chicago is an important World commodity and financial futures trading center.  It is home to the Chicago Mercantile Exchange (CME) which joined with the Chicago Board of Trade (CBOT).  It is also home to the Chicago Board of Options Exchange (CBOE) which trades futures stock options and other financial instruments.