Insulation
Insulation is a material that slows heat from moving from one place to another.
Think of it this way
A thermos has two walls with empty space between them. That gap slows heat from moving in or out. Hot soup stays hot because heat cannot cross the empty layer. The inner wall keeps the soup warm; the outer wall stays cool to the touch.
Explaining insulation by grade level
Think about an egg in a box lined with warm, soft stuff. The soft layers trap heat inside and block cold air from getting in. Without those layers, heat slips right out. The more layers you add, the longer the inside stays warm.