You are the AI. All you can store of a picture is a list of 0s and 1s — no rows, no columns, no shape. Can you still tell which digits look alike? The scary distance formula from class turns out to be nothing but count the cells where two pictures disagree, one cell at a time.
Click an empty cell to load that pair of digits above, then Step or Run the sweep — the result is recorded here. Which two digits are closest? Closest in what sense?
A digit's size here is its distance from the all-blank grid — sweep a digit against blank and you are just counting its filled cells. Click a digit to load it vs. blank.
Pick two digits (neither of them 8). Their distances from 8 are computed the same way you have been sweeping.
The Σ ("sigma") is nothing new — it is shorthand for the long sum you just watched being built, one same-or-different question per cell. Fifteen tiny questions, added up.