Algo trading or algorithmic trading (also called quantitative or automated trading) refers to the process of using computer programs to automate the process of trading (buying and selling) financial instruments (stocks, currencies, cryptocurrencies, and derivatives).
Algorithms are simply a set of rules that a computer program executes in sequence until a desired endpoint.
These computer programs are coded to trade according to these rules that have been defined for them.
In the case of algorithmic trading, the algorithms are simply a series of criteria that must be met to execute a buy or sell order.
Inputs could be based on a strategy to take advantage of different market behaviors such as the specific change of a price that triggers an algorithm to execute specific trades, or other factors like volume, time, or technical indicators.