Something is afoot when it's happening or being planned, especially something significant or mysterious. When a teacher says, “I sense mischief is afoot,” she suspects students are up to something. When a detective notices clues that a crime is afoot, he realizes something wrong is taking place.

The word carries a sense of activity and movement, often with an air of secrecy or excitement. You might hear that “big changes are afoot at school” when the principal hints at new plans without revealing details. In Sherlock Holmes stories, the detective famously declares, “The game is afoot!” when he begins pursuing a criminal, meaning the chase has begun.

While we don't use it constantly in everyday conversation, it appears often enough in books and formal speech that you'll recognize it when you see it. It's particularly useful when something is brewing or developing but hasn't fully revealed itself yet.