Expert advisor (EA)
An expert advisor is a program that runs inside MetaTrader and can analyse the market and open, manage and close positions automatically.
An EA applies rules coded in MQL4 or MQL5: entry conditions, stop, take profit, position sizing. It runs as long as the platform is open, often on a remote server (VPS) so it can work around the clock.
A robot guarantees nothing. It executes without emotion, but its rules may be poor or over-optimised on past data. Many EAs sold online show superb backtests that don't survive the live market.
Before running an EA live, test it on demo or in a forward test for several weeks, and ask for a verified live-account history rather than just a backtest curve.