Field above word count on Page Edit screen

Sorry for what is probably a very basic question. I am an extreme WordPress/I-excel newbie and I can find no references to it (it might help if I knew what it was called). What is the meaning/purpose of the small field just above the word count and below the content window on the page editing screen? It seems to usually have p in it, but it changes from time to time and I’m not sure why. If it has some useful function, could someone point me to where I might educate myself on its use?
Thank you