Hoo boy. HOOOOO boy. This one.
Determination is an energy source generated by stubbornness, Frisks, and Charas. The substance is so powerful, it can melt flesh and even warp the perception of reality. It is characterized by its blood red glow, and it sometimes boils at room temperature. Determination is used for many things, the most common of these being Resets. It is essentially the miracle substance of the Undertale universe.
The one thing it can't do is heal, because Determination is inherently evil. Yep. That's why it's used in resets so often. You can't trust that sanguine glow, can you?
Determination is often used to cheat the system, raising people from the dead and destroying whole timelines.