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What exactly are the sitting bones or sit bones?
The sit bones are literally what you sit on. The anatomical name is: Ischial Tuberosity.
A tuberosity is another word for roundness. Think of the shape of an edible tuber, like a turnip or sweet potato. In the diagram of the human pelvis (male pelvis on the left, female pelvis on the right) you will notice that at the centre is the sacrum which you have already learned consists of 5 fused vertebrae. What look like elephant ears on either side of the sacrum are the ilium. At the bottom are two round holes which look like a mask with two eye holes. At the most bottom outer surface of the eye holes are the sit bones. When sitting on a hard surface with good posture you will feel these bony prominences. Try it. You may need to pull some of your buttock flesh out of the way depending on how much buttock you have covering these bones. Tip the pelvis forward and back or side to side to become aware of these important landmarks.