Say you forgot to do your homework. You're likely to get a stern look from your teacher. Now, imagine you live on a houseboat. Your teacher's stern expression might be followed by your parent's strong suggestion that you head to the stern of your home to think over your missed assignment.
As an adjective, stern usually refers to a level of harshness, or firm adherence to an expectation (like turning in your schoolwork on time). When the word is used as a noun, stern points toward the rear of something, such as a sailing vessel.
In literature, you may run into the word stern as an adjective, a noun, or an adverb (which adds an -ly ending).
From Joseph Conrad's The Secret Sharer:
It was the mate moaning at my elbow. He was thunderstruck, and as it were deprived of the moral support of his whiskers. He clapped his hands and absolutely cried out, "Lost!"
"Be quiet," I said, sternly.
Nathaniel Hawthorne added -ness to make a noun of the root word in The Scarlett Letter:
"Peace, Hester — peace!" replied the old man, with gloomy sternness - "it is not granted me to pardon. My old faith, long forgotten, comes back to me, and explains all that we do, and all we suffer."
As seen in Treasure Island, by Robert Louis Stevenson:
At any rate, the boat sank by the stern, quite gently, in three feet of water, leaving the captain and myself, facing each other, on our feet.
From Oscar Wilde's The Picture of Dorian Gray:
"You were stern, harsh, offensive. You treated me as no man has ever dared to treat me — no living man, at any rate."