What an Expert Advisor is

An Expert Advisor (EA) is an automated trading program that runs inside MetaTrader. Once attached to a chart, it can analyse price and place, modify and close orders without human intervention.

EAs versus the other program types

MetaTrader runs three kinds of MQL5 program — knowing the difference avoids confusion:

• Expert Advisor — can trade automatically. Driven by incoming price ticks. This is what we build.
• Indicator — only calculates and draws values on the chart (a moving average, an oscillator). It cannot place orders.
• Script — runs once to perform a single action, then exits. No ongoing trading loop.

What an EA is good and bad at

An EA executes a defined rule set tirelessly and without emotion — no fear, no greed, no missed sessions, no fat-finger entries. That discipline is its great advantage.

Its weakness is that it does exactly what you coded — including your bugs and your bad assumptions. An EA cannot notice that the market regime has changed, that a news event makes today different, or that your logic has an edge case at the year boundary. It will follow a flawed rule straight into a loss, fast.

The responsibility this creates

Because an EA can place real orders automatically, a coding error can lose money in seconds. This is precisely why the testing discipline in this track is not optional. You never run untested code on a live account — and even tested code goes through demo and small-size live first, as the final chapter covers.