This is an automated program that by definition has a scenario embedded in it (trading strategy, MTS - see), that means the terminal is able to trade without a trader. The simplest advisors are built on the principle of "if ... then ...". For example, if the price goes up by 20 points, then buy, if it goes down to 25 - sell, etc. Many of the advisors are available to download for free on the Internet and the vast majority of them are easily installed into the MetaTrader 4.0 terminal (see). You can see the “Advisors” button on the top bar at the top of the page. You can learn more about the use and creation of expert advisors in the book “Mechanical Trading Systems: Pairing Trader Psychology with Technical Analysis” by Richard Weissman.