When a session of practice of any the four ‘active’ aspects of the System of Meditation is over, it is important to ‘just sit’ for some time before finishing. Just Sitting is a space of non-action in which anything can emerge. Often the fruit of the previous practice only emerges when you stop ‘doing’ it. And Just Sitting is the non-doing space in which that may (or may not) happen. Just Sitting also allows assimilation of what has just been done, and provides the necessary counterpoise to activity and effort. Just Sitting is a matter of simply ‘being’ with whatever happens in awareness, without attaching to it or rejecting it.
Especially when done as a meditation in its own right, Just Sitting enables the qualities of the previous four stages of the System of Meditation to emerge.
Here’s a great introduction to Just Sitting by Subhuti.