My English is approximartive, I'll try anyway, as your interest is Italian language 1i. In sostanza il suo discorso riguardava ABC 1e. The gist of his speech was about ABC 2i. In sostanza allora non vuoi piu` andare a XYZ 2e. all in all I see you don't want to go to XYZ anymore 3i. ma hai capito tu in sostanza cosa voleva il capo ? 3e1. did you get the sense of what the {director, boss, chief} wanted ? 3e2. did you get the essence of what the {director, boss, chief} wanted ? 3e3. did you get what the {director, boss, chief} really wanted ? 4i. in sostanza, qui nel centro di Roma, ci troviamo di fronte alla scena di un delitto anche se dell'antichita` 4e. {in short, to sum up}, here in the center of Rome, we are facing a crime scene, even if of ancient times. Summing it up, it seems to me that English language has several specialized words for our Swiss-knife word /in sostanza/; we are importing this kind of words via an heavily guarded tunnel under the Alps; Julius Caesar dug it before Brutus killed him in the center of Rome (at Roman Forum, Foro Romano).

actually "essentially" was just what I was looking for thx!

I figure you already found out about the direct translation "essentially", but look for a slang-specific translation? Maybe easier to help if you reproduce the whole sentence and add some context.