The bottom of my feet skin cracks all over very deep cracks, very painful, what is this and what should i do?

It is what it is: Very deep cracks. They are painful because they are dee. Why did you let them get so deep? Those did not happen overnight. I like using high concentration urea creams like 40-50%. However, those are prescription. So, you may want to try applying caseline on the cracks, wrapping weach foot in Saran wrap and waring a sock over this and go to sleep. Try it for a few days. If no help - see doc.

Dermatitis/eczema: See a podiatrist or dermatologist. In the mean time try moisterizing cream and avoid no socks and backless shoes.

Fungus infection: Extreme dry skin with fissures are some common causes. Go see a podiatrist.

Urea cream: Large heel cracks can be treated by debridement, urea based creams under occlusion and wearing moisture wicking socks. A trip to the podiatrist should help.