The order book is the live list of resting buy (bid) and sell (ask) orders at each price level. Depth is how much size sits at each level; spread is the gap between the best bid and best ask — a wider spread usually means thinner, less liquid conditions.
Order-book imbalance compares total bid depth to total ask depth near the current price. A heavy bid imbalance suggests more resting buy pressure than sell pressure at that moment — useful context, but not a signal on its own, since a large resting order can be pulled instantly.
AI NinjaTrader pulls genuine Kraken order-book snapshots rather than estimating this from candles — when a snapshot isn't fresh, liquidity evidence is marked unavailable rather than guessed.