The S-state cycle describes how electrons are added and donated from the manganese cluster of photosystem II, to P680*. It also demonstrates the oxidation state of the manganese ions present in the cluster.
The manganese cluster is important in refilling the electron hole left by an electron migrating away from chlorophyll, up the photosystem to plastoquinone. By donating an electron from the manganese cluster, this hole (area of positive charge) can be filled (brought back to neutral) to allow for further electrons to be transported to plastoquinone (which subsequently produces plastoquinol, which is then transported to cytochrome b6f).
As the manganese cluster becomes highly charged after the two electron holes have been filled, it is able to split water molecules. This restores the manganese cluster to its original state (as required for it to be a catalyst) and forms O2 (O=O), which is released as oxygen gas.