Jeff Mason mulls its meaning:

It is thinking of nothing in the sense of not categorizing things or making calculations about them. It is neither having abstract truths before one’s imagination, contemplating symbols or images, nor attending to sensations. ‘Single-minded’ mindfulness is neither engaged in the world, nor apart from it. It does not tell itself stories, valuing or negating, wishing or hoping, but receives and accepts whatever is going on as long as it continues; allowing thoughts and feelings, words and images, to exist as soon as they arise and to let them go as soon as they are ready to leave.