A container is a project that creates a logical grouping of projects. Containers can ease the building of large multiproject systems. You can have containers practically anywhere you want on the filesystem, with one exception: containers can't appear in the parent folders of other projects. The IDE doesn't support the creation of projects in projects.
Containers let you specify just about any number of build configurations (which are analogous to build variants in C/C++ projects). Each build configuration contains a list of projects and specifies which variant to build for each of those projects.