How Does the Concept of a “Block Race” Relate to the Generation of Stale Shares?

A "block race" occurs when multiple pools or miners simultaneously find a valid block solution. The first one to have their block propagated and accepted by the majority of the network wins the race and the reward.

Any shares submitted to a pool after a winning block has been found, but before the pool receives the new block template, become stale shares. The faster the propagation, the fewer stale shares are generated.