What is Footprint chart?
A chart that shows, for each price inside a bar, how much traded at the bid and how much at the ask, so you can see where and how aggressively trading happened within the bar.
In context
A footprint chart keeps it. It "opens" the candle and shows, for every price level inside the bar, how many contracts traded there and, of those, how many were traded by an aggressive buyer (a market buy order that lifted the ask) and how many by an aggressive seller (a market sell order that hit the bid). You learned the idea of the aggressor in earlier chapters: every trade has two sides, a passive side that was resting in the order book with a limit order, and an aggressive side that arrived with a market order and crossed the spread to trade immediately. The footprint is simply that aggressor information, sorted by price, for each bar.