When data is copied across many machines for speed and resilience, keeping every copy identical at every instant is costly and sometimes impossible. Eventual consistency accepts that copies may briefly disagree, promising only that, absent new changes, they will converge to the same value.
This trade buys availability and speed, which many applications value more than instant agreement. It suits cases where a short delay in seeing the latest value is harmless, and it is the wrong choice where every reader must see the very latest state at once.