These notes were made on an envelope that Lovecraft unfolded; he wasn’t doing too well financially in 1931, so he was saving paper. Good thing, too, since ATMoM wasn’t published until 1936.
Lovecraft made about seven pages of notes, and this one details the anatomy of the Elder Things, just one of the mind-bending creatures from the story. If you’ve never read it, you definitely should. The entire text is available here for free. You will doubtless recognize a number of elements that have been
ripped off homaged by lesser writers ever since.