Could someone explain frost heave for me? I have an attached, unheated garage, with footers that are 24" wide and 24" deep, which I understand to be below the frost line for this part of the country. The concrete garage floor is tied to the foundation with rebar. Does this protect my garage floor from movement due to frost heave?

It occurs when moisture in the soil freezes, and as it expands, the surface (e.g. your foundation/slab) is lifted by this expansion. That's why the codes call for a certain depth of pouring based upon your specific area. As long as your garage was built to code, you 'should' be fine. So, the basic answer to your question would be: Yes.