An expert advisor, usually shortened to EA, is software that places trades for you automatically once you set its rules. It watches prices, opens and closes positions, and manages stops without you touching each order. Traders use EAs to trade around the clock, to remove emotion from decisions, and to run strategies too fast for manual trading.
Firms treat EAs very differently. Some allow them outright. Some allow only EAs you built yourself. Some ban automated trading entirely, and some ban particular EA styles, such as systems that double position size after every loss. E8 Markets, for example, allows expert advisors with one strategy per user and recommends an EA you programmed yourself, while FTMO allows expert advisors subject to platform limits and legitimate trading. The EA policy is a plan-level detail, not a firm-level given, so it needs checking on the exact plan you buy.
This matters because an EA can breach an account faster than a human. A faulty setting can open dozens of trades in minutes, and a strategy that worked in testing can behave differently on the firm's prices and spreads. The firm judges the result, not the intention, so an EA error counts the same as a manual error.
The common trap is buying or downloading an EA and running it without reading the firm's automation policy. We recommend confirming three things first: whether EAs are allowed at all, whether outside EAs are allowed or only your own, and whether the EA's style is banned. Prop trading can cost you every fee you pay.