The equation by itself claims that mass depends also on the content of energy where mass must be strictyt understood as the inertia of a body i.e its capability to withstand a change of speed
Einstein wrote

Ist die Trägheit eines Körpers von seinem Energieinhalt abhängig?"
(Does the inertia of a body depends on its content of energy ?)

Part of the mass (Inertia) of a body depends on the amount of "Matter" and part on the amount of "Energy"

How much ist the contribution of "Energy" and how much is the contribution of "Matter" to the total "Mass" ?

The equation by itself does not provide any answer to such question