Level streaming Question

Whats is level streaming and level loading, and which should you use for an open world shooter game? Im creating an open world shooter. The world is divided in to separate areas so should I use level streaming on each area or just make one big map with everything. I dont know the difference please explain.

Level Streaming is used when your map is too big to store it in memory. It allows you to load and unload sublevels when you need.

Level Streaming is very well explained in UE documentation. There are also a bunch of tutorials on the youtube.