Containers are nodes that contain other nodes, using them is a great way for graph spaghetti prevention and to keep your graph tidy.
This node has to two sub nodes 'IN' and 'OUT', those should be there as long as
this node is here. To add more ports go to 'IN' or 'OUT' node, click on the info button.
When you delete this node, all containing sub nodes will be deleted.
You may create any node in the container but you cannot create containers inside containers.