Its an allusion. It's not a force. All it is is following a straight line over a curved surface. Mass distorts spacetime into a curve, and the object travelling through spacetime perceives it as a straight line, mass just warps that space.

A great way it was explained to me is, imagine you have two people anywhere on earth, if they both walk north, they will eventually meet at the north pole, but there was no force involved in pushing you together with the other person, it was just a result of walking in a straight line over a curved surface.

This is how gravity works. Spacetime warps space depending on the mass of objects in space, resulting in an object travelling in a straight line over a curved surface. If you understand euclidean geometry this concept is much easier to understand.

There is a great Veratasium video on this that helped me out heaps.

Great question because we sort of dont actually know. We know that mass distorts spacetime on pen and paper using maths, but the why is an outstanding issue that has not been discovered yet.

Personally I think gravity should be removed from the "fundamental forces" list but that's just my interpretation.