"Really, it is easy."
Translation:Vere, estas facile.
If you look at the Esperanto sentence, there is no "it" here. Ĝi is not used in this sentence.
Therefore this sentence is not about some specific it (object) being easy. It is just a general statement, just like "It's raining" = pluvas. No it (dummy pronoun) is being used.
And when you have a general statement the "e"-ending is used instead of the "a"-ending.
Just like when your friend comes over to you and says "I finished the Esperanto Duolingo course" you say "Bone!" not "Bona!".