Thermite Paste is a chemical used by the UNSC for stealth missions where soldiers may encounter steel doors or obstacles that they will need to get through quietly.
The paste is likely comprised of iron oxide, used specifically to "burn" aluminum metal to make aluminum oxide, as in a typical Thermite Reaction. It is applied to the surface of the obstacle and then lit. Once the paste is ignited, it produces a huge amount of heat and light, and when it extinguishes itself, a hole has been punched through the steel.
The Spartans have used thermite paste occasionally to infiltrate human structures, most notably during The Battle of Sigma Octanus IV.