A scrypt is a hash calculation system. Either SHA-256 or scrypts are used for hashes in mining across nearly all virtual currencies. For example, Bitcoin uses SHA-256 and Litecoin uses scrypts. Scrypts are less complex than SHA-256 so they need less memory allocation due to not requiring another 1,000 high speed calculations that can only be viably done on an ASIC instead of a CPU.
This allows mining to be done on a CPU.