Imagine a TV showing a video of a bizarre, unfamiliar object - let's call it a squirgle. The video was computer generated by a one-time piece of code, so there's no "real squirgle" somewhere else in the world which the video is showing. Nonetheless, there's still some substantive sense in which the squirgle on screen is "a thing" - even though I can only ever see it through the screen, I can still:

Notice that the same squirgle is shown at one time and another time.
Predict that a shiny spot on the squirgle will still be there later.
Discover new things about the squirgle, like a scratch on it.

… and so forth. The squirgle is still “a thing” about which I can have beliefs and learn things. Its “thingness” stems from the internal consistency/compressibility of what the TV is showing.