On the sense-datum theory, a perceptual experience in which something appears F to one consists in a relation of perceptual awareness to something which is actually F (Level 1). So whenever a subject has a sensory experience, there is something of which they are perceptually aware. This relational conception of experience is sometimes called an “act-object” conception, since it posits a distinction between the mental act of sensing, and the object which is sensed.
A sense-datum theorist calls the object of an experience a sense-datum. We can thus re-formulate the Phenomenal Principle espoused by the sense-datum theorists in these terms:
If there sensibly appears to a subject to be something which possesses a particular sensible quality F then there is something—a sense-datum—of which the subject is directly aware which does possess that sensible quality.
For the sense-datum theorist, the character of an experience is somehow explained (at least in part) by the sensible qualities of the sense-datum one is aware of. (Level 2). Consider an experience which one would describe in terms of seeing a snow covered churchyard for what it is. We can isolate certain aspects of the phenomenal character of such an experience, such as the appearance of whiteness to one. We want a theory of experience to explain such aspects. The sense-datum theorist will claim that things appearing white to you consists in your perceptual awareness of a white sense-datum. The character of your experience is explained by an actual instance of whiteness manifesting itself in experience.
Now ultimately the sense-datum theory opposes our ordinary conception of experience, but it doesn’t as we have it so far. For at the moment we have no opposition between sense-data and ordinary objects. Suppose that one has an experience of a churchyard as described above, and so one is perceptually aware of a white sense-datum. Now suppose one’s experience is veridical. Well, for all we’ve said, the sense-datum one is aware of could be an ordinary bit of mind-independent reality: some white snow. This is not ruled out, for a sense-datum is just whatever it is that one is aware of in a perceptual experience which instantiates the sensible qualities which characterize the phenomenology of one’s experience. All we know about sense-data is that they must satisfy two conditions:
sense-data are objects of direct awareness; and
sense-data bear sensible qualities.