The quote states that the thoughts, emotions, and mental narratives that constitute your 'mind' are not your fundamental identity. You are not the stream of consciousness itself, but rather the underlying awareness that perceives this stream. This is the surface meaning: the observer is distinct from the observed.

In the context of modern non-dual teachings, drawing from traditions like Advaita Vedanta, this points to the distinction between the empirical self (the ego, identified with the mind) and the true Self (Atman), which is pure consciousness (Brahman). The mind, with its incessant thinking and conditioning, creates the illusion of a separate, limited individual. The 'awareness behind the mind' is the formless, unconditioned consciousness that is always present, the silent witness to all mental and sensory experience. It is the unchanging ground of being, the 'I Am' presence that Tolle often speaks of. By shifting identification from the mind to this awareness, one realizes their inherent wholeness and freedom from the suffering generated by mental identification.

In your life: When you notice yourself caught in a cycle of anxious thoughts or self-criticism, pause and feel the space of awareness in which these thoughts arise and pass. Recognize that you are not those thoughts, but the silent presence observing them.