In Einstein’s 1905 paper entitled: “Does the Inertia of a Body Depend Upon its Energy Content,” he concluded that: “the mass of a body is a measure of its energy-content.”

What exactly did he mean by the terms ‘inertia,’ ‘mass’ and ‘energy content’?

He was talking about the mass of a particle being reduced by an amount proportional to the light emitted from it (when light is emitted), where light is a form of energy. And since light can be converted into other forms of energy, the general concept holds.

Inertia: The empirically observed phenomena that an object resists a change in its motion. In the older interpretations, results in Newton’s 1st law: an object in constant motion stays in constant motion unless acted upon by an outside force.

Mass: A measure of this inertia. Under a different interpretation (post-Einstein) mass could be simply a factor utilized to achieve covariance. (Regardless, in the simplest of pictures it still measures inertia). Using a different definition of “distance” in the Einstein-Lorentz relativity and especially Einstein’s general relativity, this mass can be interpreted to be increasing as a function of velocity, approaching infinity as the velocity approaches the speed of light. An object’s mass at rest is dubbed its “rest mass.”

Energy content: A measure of the total energy of the system.
-Similar to previous responder: he’s concluding the famous E=mc^2 for the rest mass of an object.
As many people don’t realize, the fundamentals of even physics are rooted in ontology. Hope this helps.