Leading span A is the midpoint of tenkan-sen and kijun-sen; leading span B is the midpoint of the highest high and lowest low over fifty-two periods. Both are shifted forward in time, which is what gives Ichimoku its distinctive forward projection.
The forward shift is often misunderstood as prediction. It is not: the values are computed entirely from past data and then drawn ahead, so the cloud you see in front of price is already fully determined by history.
When span A is above span B the cloud is bullish and usually shaded accordingly; when they swap, the cloud twists. That twist is the earliest visible change in the system, but again it reflects data already in hand.
Related: kumo-cloud, tenkan-sen, kijun-sen, ichimoku-kinko-hyo, chikou-span