Two principles, stated with the precision of an axiom. The unreal has no existence. The real never ceases to exist. The seers of truth have arrived at the conclusion of both. This is one of the Gita's most concentrated philosophical statements — every word carries weight.
What is 'unreal' here? The body, the material world as we experience it — always changing, always decaying, never staying the same from one moment to the next. What is 'real'? The soul — changeless, deathless, untouched by time. The body is like a wave on the ocean. It rises, crests, dissolves. But the ocean remains.
Krishna adds an important qualification: this is not his private opinion. The seers of truth — tattva-darshis — have seen this. The word 'seen' (drishta) is deliberate. This is not theory arrived at through argument. It is direct perception by those who have looked deeply enough to tell the permanent from the passing.