What is Algo Trading button?

What it means

It is a single toolbar toggle, green when enabled and red when not, and it overrides every per-EA setting. An EA can be attached, loaded, running its logic and generating signals while this button is off, and it will place no orders at all.

Why it matters

The state does not always survive a restart, and some MetaTrader builds reset it after an update. This makes it the first thing to check whenever a previously working setup stops trading — far more often than not, that is the answer.

What this changes in practice

A related trap catches people who have enabled it: the EA also needs "Allow Algo Trading" ticked in its own properties dialog, and both must be true. See AutoTrading is disabled for the full checklist.