In other words, what we're thinking about — what we're focused on — filters the world around us so aggressively that it literally shapes what we see. So, Drew says, we need to think carefully about the instructions we give to professional searchers like radiologists or people looking for terrorist activity, because what we tell them to look for will in part determine what they see and don't see.
Here's the original "Invisible Gorilla" video:
The journal article reporting this experiment is available as a pdf.
And here is the same researchers' door trick (illustrating change blindness):