A sophomore is in their second year, either in high school or college. Once you became a sophomore, thinking you now knew everything, you pitied the freshmen for their confusion over how to write college papers.
Sophomore comes from the word sophism which means "a bad argument that's clever but false," and that's sometimes how sophomores in college act. They're not bright-eyed newcomers anymore, and think they know a lot, but really they've much to learn. We also use the word for other seconds: a band's second album is usually called their sophomore album.