You use essentially to emphasize a quality that someone or something has, and to say that it is their most important or basic quality.
It's been believed for centuries that great writers, composers, and scientists are essentially quite different from ordinary people.
几个世纪以来人们都认为伟大的作家、作曲家和科学家本质上与常人大不相同。
You use essentially to indicate that what you are saying is mainly true, although some parts of it are wrong or more complicated than has been stated.
His analysis of urban use of agricultural land has been proved essentially correct.
他关于城市占用农用土地的分析已经被证明基本上正确。