What do the philosophers Descartes and Locke think it means to be conscious?
Due to the fact that formatting only allows for brief answers, I had to shorten your question because Kant's views on consciousness are far more complex than space allows.
According to Descartes, thought relates to consciousness. Descartes sets forth in The Principles of Philosophy, Part I "The Principles of Human Knowledge," that thought relates to "everything which happens in us while we are conscious, in so far as there is consciousness of it in us." Since it is difficult to understand humans as being conscious of thoughts as opposed to being conscious of the objects that the thoughts represent (in other words we are more conscious of a ball than we are conscious of our thought of a ball), Descartes takes it one step further and says that ideas are any thoughts that make us aware of thoughts. Therefore, we are aware of the idea of a ball, which alllows us to have a thought of the ball. Hence for Descartes, consciousness is the perception of an idea and the cognitive reflection that makes a person aware of the thought.
On the other hand, Locke, in his An Essay Concerning Human Understanding, defines consciousness as personal identity. Consciousness allows a person to use reason and reflection to "consider it self as it self" (Ch. 22.11 "Personal Identity"). Similar to Descartes, Locke argues that "one cannot think without being conscious of it" (Ch. 22.15 "Whether in Change of thinking Substances there can be one Person"), however, he also argues that we are not conscious at all times, such as when we are sleeping.
Hence, while Descartes defines consciousness as thought and the awareness of thought, Locke defines consciousness as an awareness of self, which stems from the ability to think.