WHAT IS AN EXPERT ADVISOR?
An Expert Advisor is a computer program written in the MQL programming language.  
After an Expert Advisor has been correctly written and compiled, it is attached to a price chart on a MetaTrader account.  The Expert Advisors resides on and runs on your local computer.
An Expert Advisor’s logic is run each time new price data is sent from your broker’s server to your computer.  The Expert Advisor recalculates indicators and executes its logic using the new price data.  
The EA is capable of doing anything programmatically that can be done manually, such as opening new positions and closing or partially closing positions.