Two birds sit on the same tree. One eats the fruits — sweet and bitter, one after another. The other simply watches, luminous and still.
This is one of the most famous metaphors in all of the Upanishads. The tree is the body. The eating bird is the individual self (jiva) — tasting pleasure, pain, gain, loss. The watching bird is the Atman — the pure witnessing awareness, untouched by any experience.
We go through life as the eating bird, forgetting that the watching bird is also us — our deepest nature. The path of spiritual practice is to recognise the witness within.