Just Sitting is one of three practises, Sangharakshita ( founder Triratna Buddhist Order ) introduced for people who are new to meditation.
It is a Samatha ( calming ) practise.
Just Sitting, is what it says on the tin, you just sit. You are not trying to do anything, you are not trying to not do anything.
While Just Sitting you are just noting what is in your experience. You are not looking to change it.
The method is to be aware of the present moment, not trying to change it, run from it or cultivate it. Just be open, just be.
At the end of a period of meditation, a period of making effort, you need time and space to allow the fruits of that effort to ripen. So it is a good idea at the end of say Mindfulness of Breathing or Metta Bhavana to Just Sit.
Just Sitting is probably more difficult than Mindfulness of Breathing or Metta Bhavana. In these two practises you have a defined focus or aim. In Just Sitting there is the very real danger you will just drift off, day dreaming the meditation away.
Wildmind by Bodhipaksa, Windhorse Publications
Buddhist Mediation by Kamalashila, Windhorse Publications
the Body by Paramananda, Windhorse Publications
the Breath by Vessantara, Windhorse Publications
the Heart by Vessantara, Windhorse Publications
metta by various authors, Windhorse Publications
The Mindful Manifesto by J Heaversedge and E Halliwell, Hay house