Grit is a coarse, gravelly dust. If you don't rinse dried beans before you cook them, you may end up crunching grit between your teeth.
Grit is small, sandy pieces of rock or stone. Another way to use the word grit is to mean a determined, courageous attitude. If you have grit, you'll keep trying to climb a rock wall no matter how many times you slip and fall. Charles Portis's 1968 novel "True Grit" tells the story of a young girl in the 1800s who embodies this quality, persevering through extreme difficulty and struggle.