Reading an airfoil
A wing has a plan shape and it has a cross-section. The plan shape is what you see from above and it is what unit 04 works on. This unit is about the other one: the curve you would see if you sawed the wing through and looked at the cut face.
NACA 4-digit sections are the ones worth learning first, because the four digits are not a catalogue number. They are the shape. Two curves are added together: a thickness distribution, symmetric about a line, and that line itself, which may be curved. The digits set both.
Thickness first
Set the first digit to zero and the section is symmetric: the line it is built around is the chord, dead straight. Now the only question left is how fat it is, which is the last two digits. NACA 0012 is 12% of the chord thick. Drag the thickness slider and watch where the fattest point sits: it does not move. The 4-digit thickness distribution always peaks at 30% of the chord, whatever you set.
One number in the readout is worth stopping on. The trailing edge does not close. At 12% thickness it is 0.25% of the chord thick, and that is not a rounding error in the drawing: the original NACA polynomial simply does not return to zero at the trailing edge. Foam and balsa will close it for you. A mesher will not, which is why the code has a sharp variant that tweaks one coefficient.
Then camber
Raise the first digit and the line the thickness is wrapped around stops being straight. That line is the camber line, and it is drawn in yellow below. The first digit is how high it rises, in percent of chord. The second is where along the chord it peaks, in tenths. So 2412 is 2% camber peaking at 40% of the chord, wrapped in a 12% thickness distribution.
Set camber to 2% and thickness to 12%, then move the second digit alone. The section barely changes and the camber line changes a lot. That is the useful thing to notice here: the second digit is a small change in shape that you will find is not a small change in behaviour once unit 03 puts numbers on it.
What a shape cannot tell you
Everything above is exact. The 4-digit family is two algebraic curves, so there is nothing being approximated and nothing to converge, which is why this page needs no solver and no download.
It also means you cannot yet answer the question you actually have. How much lift, at what angle, for how much drag, and where does it stall. None of that is in the geometry. It comes out of a viscous calculation at a particular Reynolds number, and Reynolds number is why unit 01 asked for a speed and a rough chord. That is unit 03.