Food-grade methylcellulose is a modified cellulose used to thicken and bind mixtures. Certain grades form gels as they heat and soften again as they cool, making them useful in plant-based foods and modernist cooking.
Different grades have different hydration and gelling behavior. Use the exact food-grade product specified by the formula, not a product intended for crafts or construction. It behaves differently from gelatin, which typically sets on cooling.
Reference: Further reading.