It’s used for “type aliases”. For example:
type StringOrNumber = string | number;
type DictionaryOfStringAndPerson = Dictionary<string, Person>;
Reference: (edit: removed outdated link) TypeScript Specification v1.5 (section 3.9, “Type Aliases”, pages 46 & 47)
Update: (edit: removed outdated link) Now on section 3.10 of the 1.8 spec. Thanks @RandallFlagg for the updated spec and link
Update: (edit: deprecated link) TypeScript Handbook, search “Type Aliases” can get you to the corresponding section.
Update: Now it’s here in the TypeScript Handbook.