That he is sentient ie that he thinks.
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Short answer: Descartes' doubt was like WWI - the doubt to end all doubt. Only by doubting everything could Descartes hope to find anything that was certain (even if the only certainty is that nothing else is certain!).
Descartes found it impossible to doubt his own existence. The reason for this was that he felt that thoughts had to come from himself.
Philosophically, Descartes was concerned with the existence of reality.
he believed if you can doubt it, it must not exist.
For Descartes, this was the fact that he existed as a thinker. He was perfectly sure he was thinking; and he was likewise convinced that to be thinking he had to exist as some kind of substance.