Level 2
Level 2 is the order book for a stock: every displayed bid and offer at each price, broken out by the exchange or market participant posting it. Level 1 shows only the best bid and the best offer; Level 2 shows the depth behind them.
Why it matters to a small-cap momentum trader
On a thin small cap, Level 2 shows where the supply is. A large offer at a round number can cap a move until someone buys through it; bids stacking up under the price can act as support, and when they pull away the support goes with them. Traders watch the spread, the size at the best bid and offer, and how the book reacts when price reaches a level. Level 2 is read together with time and sales: the book shows what people say they will trade, the tape shows what actually traded.
What Level 2 does not show
Level 2 shows displayed orders only. Hidden and reserve orders, and trades executed away from the exchanges, do not appear on it, and displayed size can be cancelled in an instant. Read it as evidence, not a promise.
Level 2 in Hindsight Markets
The Level 2 window shows that day's full book, exchange by exchange, with your own orders marked at their price and SSR, borrow, halt and LULD tags in its header. A marketable order fills against the displayed size, best price first, and a limit order joins the back of the real queue at the exchange you route to.
Practice this on a real past day in Hindsight Markets
Open a real past day, watch the book on a small cap as it runs, and practise reading where the size really is.