System streaming is the process of controlling input and output data streams, that is, information to and from your game. The general concept is to stream data to memory when needed based on information requests. Animations, for example, are usually memory hogs. Having animations loaded into memory before they are needed is a sure way to quickly consume all of a system's available memory.
Although Lumberyard handles much of the streaming processes, game developers using Lumberyard to build their games should be familiar with this concept and the necessary interfaces, requirements, files, and applications associated with system streaming. Lumberyard's documentation provides some cursory information regarding streaming.