Essentia is the primordial "Life-force" that makes up the universe, or to say it in another way: It is the strings that harmonize the inhabited world. To be able to "resonate" and know how "touch" this strings is to be able to manipulate and influence reality, matter, and even the minds of others.
Unbound generally make the comparison that the world is a great instrument where they can play any and even create new notes and songs thru their Schemata, but with the disadvantage that they need to know how in order to not produce vulgar acts of Anacorde. While the other supernaturals are only able to approach this instrument in a fixed delimited way thru codified, often inherited, taught or bestowed abilities called Disciplines or thru another medium like with a Facade.