What the proof does
The proof is a room in your font. It is part of Pro.
What it reads
After a build, the proof looks at each character in your font and compares it with the rest of your hand: its size against your other letters, where it sits on the line, how it looks next to its neighbors. A character that stands out from the others is noted.
What you see
Each note shows the character in real words, so you can judge it the way it will be read. Under the words, the note says what it noticed in one line: too small, sitting low, heavier than the rest.
What changes
Nothing, unless you choose. Every note has two doors: keep it as it is, or write that character again. If you write it again, your font uses the new one from the next build. The old version is kept.
On a Free account
The proof's notes, and the room, are part of Pro. A month of Pro opens them for every font you have.