What does it mean that gravity is not a force, but it is math?

Not so silly as he thinks.

Einstein's theory of gravity improved on Newton's force based concept;

Einstein's theory gave accurate results where Newton's theory failed, and it predicted phenomena that Newton's theory did not explain.

It was based on the revelation that acceleration and gravitational fields are equivalent so what we think is force can be reinterpreted. Large masses, (and also concentrations of energy equivalent to mass), significantly distort the curvature of spacetime in their vicinity and what seems to us to be the curved path through space of say a moon about a planet, is actually a geodesic, (a shortest path route), through four dimensional spacetime. This is nicely summed up by the sentence:-

Spacetime tells matter how to move; matter tells spacetime how to curve.

The concepts are counter-intuitive ,and are best described by the complicated mathematics of General Relativity. At the heart of this are the ten Einstein field equations that describes the fundamental interaction of gravitation as a result of spacetime being curved by mass and energy. Read and understand more

https://en.wikipedia.org/wiki/Introduction_to_gene...

So now you know what was meant by "gravity is not a force, but it is math".

To have a glimpse of some of that mathematics see Einstein's field equations here:-

https://en.wikipedia.org/wiki/Einstein_field_equat...

It means whoever said this statement has no clue what they're talking about.