ICE stands for Information Concealment Engine. It is a 64-bit private key block cipher, in the tradition of DES. However, unlike DES, it was designed to be secure against differential and linear cryptanalysis, and has no key complementation weaknesses or weak keys. In addition, its key size can be any multiple of 64 bits, whereas the DES key is limited to 56 bits.
Valve implemented ICE as a simple, and effective way of tamper-proofing a script file (using a 64-bit (8 bytes) key). You can ICE encrypt any file, as long as you have access to the code that loads it into the game.