Lead Foil is a material used by the UNSC for ships that traverse Slipspace. Lead foil is essential to Slipspace, and to deep-space travel in general, as it absorbs radiation and prevents it from harming the occupants of the ship. Lead foil is relatively fragile and prone to melting at extreme temperatures.
Covenant ships apparently do not use lead foil, particularly Spirit dropships, which are not capable of navigating Slipspace. However, lead foil can be installed in them to achieve the desired effect. Blue Team did this to their dropship aboard the Ascendant Justice shortly before Operation: FIRST STRIKE.