There are many types of Buddhist meditation practice: mindfulness, concentration on a single object, visualization and cultivating positive emotion, among many others. These practices all have a venerable history and purpose. In the Soto Zen tradition, there is a practice called shikan-taza, usually translated as "just-sitting," which is different from all of these. Over time I have come prefer the term "just-awareness" to "just-sitting," since I feel that "just-awareness" better captures the living beauty of the practice. The Mahamudra tradition, with which I am somewhat familiar, has a practice called "resting in true nature," which I believe is a practice similar to just-awareness.
Just-awareness is essentially an all-inclusive experience that includes and accepts everything that comes into awareness, by resting in awareness itself. It does not focus on any object of concentration, although it does require the stability that comes from the practice of concentration. Most meditation implies or requires a subject and an object. Just-awareness is different than this. As Tibetan Buddhist teacher Ari Goldfield says, in discussing this practice, "Subject and object are undifferentiable." One of my early Zen teachers liked to call just-awareness "settling the self on the self." This phrase is rather cryptic unless you understand that the first "self" in the statement refers to our ordinary personality or ego, while the second self is our "big" self -- the awareness which itself has no color or shape, but which includes everything.
Sometimes just-awareness is inferred by describing what it is not. My teacher Shunryu Suzuki liked to say, "Our effort in meditation is to get rid of something extra." What he meant is that everything you can think of or imagine is extra. When you are relaxed and open enough for all of those extra things to fall away, just-awareness is simply there.
Another time Suzuki said, "When you are in shikantaza, you don't know exactly who you are. But as soon as you get up, you know who you are again." This is a more subtle instruction. It means that just-awareness is not a particular state of mind -- certainly it is not an "altered" state. It is simply being awake, and nothing else. So when Suzuki says, "You don't know exactly who you are," he means that you are not involved in ordinary thinking or in perpetuating your personal story. You are simply resting in the great space of awareness.
The Tibetan term "resting in true nature" points to another aspect of just-awareness. It is the essence of who we really are, and who everyone really is. Just-awareness is not something to acquire or attain, it is our birthright -- always accessible, always available. At the same time we don't know it or experience it unless it is pointed out to us and unless we actually practice it. It is said that just-awareness momentarily appears when we are exhausted or falling asleep -- times when we completely let go.
From one point of view Buddhist teaching seems complex and esoteric. But when you meet someone like Suzuki who lives that teaching, Buddhism looks straightforward and simple. To all outward appearance Suzuki was ordinary and uncomplicated. In fact his students were the ones who were complicated -- full of thoughts, ideas, questions and confusion. Suzuki laughed easily, and seemed always to be attentive and available when we needed him. He didn't teach just-awareness as an intellectual thing. He eased us into it by manifesting it in his demeanor.
Once, during a retreat, a student approached Suzuki and said excitedly, "My mind just opened up to incredible spaciousness. I feel like I am one with the whole universe, with everything! Is that enlightenment?"
"Perhaps," Suzuki replied. "Oh, incidentally, how is your work in the kitchen going?"
Suzuki wasn't making light of the student's experience. He acknowledged that something had happened. But at the same time he pointed out that living enlightenment is the just-awareness of each moment and each activity, each potato or carrot. Just-awareness is so simple that it is hard for us to see it. It is not actually a thing to be seen, but a reality to be experienced. It is the deepest ease and rest of all.