Serif
A small finishing stroke at the ends of letters, as in Georgia or Times.
See it in Typography pairing. Pick Playfair + Source Sans and compare the heading's serifs with the plain body text.
In plain words
Look at the ends of the strokes in a letter like H. In some typefaces they finish with small feet or flicks. Those are serifs. They come from letters carved in stone and written with a pen. Serif typefaces often feel classic or bookish, and many books use them.