MetaTrader 5, usually shortened to MT5, is a trading platform made by MetaQuotes and licensed to brokers, who offer it to their clients under their own server. It is free to the trader, runs on Windows, with versions for macOS, Linux, web browsers and phones, and it connects to a single broker account at a time. It succeeded MetaTrader 4, which many brokers still offer, and the two are not compatible: an EA written for one does not run on the other.
The layout
The main window holds the charts. Around it sit three panels. Market Watch lists the instruments the broker offers with their current bid and ask, and right-clicking a symbol and choosing Specification shows its contract details. Navigator lists accounts, indicators and Expert Advisors installed on the platform. The Toolbox at the bottom shows open positions and pending orders under Trade, the account's closed trades under History, and the Experts and Journal tabs, which record what any running EA is doing and any errors it meets.
Account figures, balance, equity, margin, free margin and margin level, appear along the bottom of the Trade tab.
What MT5 adds over MT4
MT5 offers twenty-one timeframes against MT4's nine, a deeper economic calendar, a strategy tester that can test several instruments together, and the choice between netting and hedging account types, which has its own page. Its programming language, MQL5, is more capable than MQL4, and its Market and Signals services are built into the platform. For a trader running an automated strategy across several markets, MT5 is the sensible choice, and new EAs are increasingly written for it alone.
Where an EA lives
An Expert Advisor is a compiled file with the extension .ex5, stored in the platform's data folder under MQL5, Experts. Once there, it appears in the Navigator and is attached to a chart by dragging it on or double-clicking it. Its name then shows in the top right corner of that chart. An EA runs only while the chart it is attached to is open and the platform is running, which is why a VPS is used.
The settings that matter
Two switches control whether an EA can trade. The Algo Trading button on the main toolbar enables automated trading for the whole platform. In the EA's own properties window, opened by double-clicking its name on the chart, the Common tab has "Allow Algo Trading", which must also be ticked. If either is off, the EA runs but places no trades, and the Experts tab will say so.
The same properties window has an Inputs tab, where the EA's settings live. Any documented setting, such as a risk level or a mode, is changed here. Settings can be saved as a .set file and loaded again, which is worth doing before any change.
You install an EA, attach it to a chart, and nothing happens for a day. The Experts tab shows the line "automated trading is disabled". The Algo Trading button on the toolbar is off. You turn it on and the EA's next signal is placed. Five minutes with the Experts tab would have saved the day.
The Experts and Journal tabs are where an EA explains itself. A well-written EA logs each decision, each order, and each error, including the broker's reason for rejecting an order, such as insufficient margin or a volume below the minimum. When something looks wrong, the log is the first place to look and usually the last.
Keep MT5 itself up to date, since brokers update their servers and old builds lose features, and keep only the charts the EA needs open, since each open chart consumes memory on a VPS.