In malware research, this refers to a program that makes malware hard to read by researchers. The crudest technique for crypters is usually called obfuscation. A more elaborate blog post on that is Obfuscation: Malware’s best friend. Obfuscation is also used often in scripts, like JavaScript and VBScript. But most of the time, these are not difficult to bypass or de-obfuscate. More complex methods use actual encryption. More information about this and related subjects can be found in our blog post, Explained: Packer, Crypter, and Protector