What distinguishes the thoughts of a human being from the thoughts of other animals is that the latter don’t think about their thoughts.
For example, my cat occasionally believes that a mouse is within striking distance. She doesn’t ask herself whether this belief is true or false, or whether the evidence justifies her belief. She merely believes.
Something similar is true of her inclination to pounce on the mouse. She doesn’t ask herself whether attacking and killing the mouse is morally permissible, or not. She merely attacks and kills.
Descartes’s thought – “I think, therefore I am” – is reflexive. Not only does Descartes think, he understands that he thinks, and that other thoughts follow from what he thinks – or so he thinks, and hopes.
As for what Descartes meant, that’s a long and controversial story. The consensus seems to be that it’s impossible to say precisely what he thought, because he was thinking many different things. He might have thought that “I think, therefore I am” was a logical inference. He might have thought it was a self-evident intuition. He might have confused the two.
