Saturation is a term used to indicate that a particular stake pool has more stake delegated to it than is ideal for the network, and once a pool reaches the point of saturation it will offer diminishing rewards. The saturation mechanism was designed to prevent centralization by encouraging delegators to delegate to different stake pools, and operators to set up alternative pools so that they can continue earning maximum rewards. Saturation, therefore, exists to preserve the interests of both ada holders delegating their stake and stake pool operators.
The goal is to avoid any single pool becoming too large – thereby disincentivizing delegation to other pools – and receiving a disproportionate amount of the rewards. The health of the network is partly determined by having a high number of active stake pools with a balanced amount of stake delegated to them. The more numerous and geographically diverse the network’s pools, the better.