A slice is a narrow piece or portion of something (particularly food), like a slice of bread or a slice of pizza.
You can order a slice of cake or pie at your favorite bakery, or fry potatoes after cutting them into thin slices. Slice can also be used figuratively: "His brothers each own a slice of the business." The expression "slice of life" usually describes a realistic version of life that's depicted in a book, movie, or play. And in golfing, a slice is a stroke that makes the ball curve sharply to one side.