I am self-taught and have successfully coded simple collisions of planes, spheres, rays and up until interactive cloth simulations so I do have some basic sense of which part of the physics goes where in code; but no idea of the terminologies.

It's the jargon that I do not understand: What is a physics solver? How does a solver look like in code or which part of the engine is considered a solver?

For example what does the phrase "Box2d is using iterative solver" mean?