ECN which stands for "Electronic Communication Network" is a professional trading account. With this type of account, you have access to an automated system that matches buy and sell orders in the FX market. The ECN  is the organization of the trade of financial instruments by a big number of operators in one electronic system that enables all the trading participants to interact directly with each other wherever they are located geographically.
At the same time all orders for selling and buying are available and displayed to the trade participants. In this case, the spread is the difference in price between the best orders of salesmen and buyers, a new trading participant can place his order within this spread, thus changing the upper or the lower price limit.
 The ECN makes money by charging a fee for each transaction. The spread can vary between 0 when the market is calm to a high number of pips when it becomes volatile when market indicators or news are being published.