Hack happens to be a programming language which Facebook came up with, where they combined elements of static-type programming languages such as C with dynamic-type languages like PHP. Whenever one codes in a static programming language, it will inform you of an error prior to the program running, while the forward looking, dynamic type language that one codes will continue until a program crash happens.
Hack intends to deliver its own way of catching errors before running the program starts to make sense, which could eventually be the springboard for developers to code faster than ever before. After all, speed in coding is also essential, although whether it could be on part with writing code that runs faster remains to be seen. [Press Release]