As the topic states. I don't understand what is meant by a 'standalone' ECU

It works by itself, with no need for the original computer. there are 4 types of aftermakert ecu's. stock ones with chips, apex'i power fc which takes the place of the stock ecu completly but uses the factory wiring, ones like haltech that use it's own ecu and wiring, and the fourth kind works together with the stock ecu