In the context of trading automation, order flow describes the step-by-step path a signal takes from the moment a chart condition is met to the moment an actual order reaches a broker for execution - the alert firing, the message being sent, the receiving system interpreting it, and finally an order being placed, with the instrument, quantity, and order type all needing to be handled correctly at each step.
Related Reading
See this full path traced out in Understanding Order Flow in Automated Trading.