Time and sales, often called the tape, prints each fill as it happens. It shows what actually traded, while the order-book shows only what people are willing to trade.
Reading the tape means watching whether prints are hitting the bid (aggressive sellers) or lifting the ask (aggressive buyers), and how large they are. It is a skill built on volume and speed, and it is easy to see patterns that are not there.
Example: a burst of 500-share prints at the ask for thirty seconds, with the price ticking up, suggests urgent buying. The same prints at the bid suggest urgent selling.
Related: order-book, level-2, volume, fill