In Newtonian physics, you can think of every object as having an inertial mass (m_i), which controls how it responds to forces, and a gravitational mass (m_g), which describes how strongly it pulls on other masses through gravity. The gravitational mass can be thought of as the "gravitational charge," in analogy to electric charge.

In Newtonian mechanics, there is no reason for m_i to equal m_g. It is simply a coincidence. If they were not always equal, then some objects would fall faster than others in a vacuum.

The interesting thing is that there exists a class of forces which are proportional to mass - so-called "fictitious forces." These are forces like centrifugal force, which only appear to exist if you are in an accelerating frame of reference. You only feel centrifugal forces if you're spinning, for example, and the "force" you feel is just your body trying to keep on going in a straight line. Fictitious forces always act on an object with a force proportional to its inertial mass.

Einstein's insight was essentially to view gravity as a fictitious force. The hint is that m_g = m_i. In General Relativity, gravity is not a "real" force. The effects of gravity are caused by geometry being non-Euclidean.

If you want to understand this better, you can start by reading about the "Equivalence Principle": https://en.wikipedia.org/wiki/Equivalence_principle