It is the most popular word among student community. Teacher of every subject asks her students whether her lesson is understood by them or not. If more students understand their subjects, more are benefited. Even teacher also will get credits when more number of her students understand her teaching!
Between a wife and husband this word plays an important role and paves a beautiful path for their life when understanding continues till the end.
In case of common citizens, understanding and observing statutory rules makes them and their society to lead a comfortable life.