When we say discipline trading it means just follow the rules strictly. Forex trading is the name of a business and for doing this business we need to follow some rules, if we follow these rules we are successful and if we don't follow these rules then we are losers. So discipline trading means just follow the trading rule sand don't violate any rule.
For example if you open many trades and open big lot size trades then you will soon get a margin call and I will say that you are not a discipline trader because you have violated the forex trading rule.