Traditionally, this phrase describes a compiler that turns Javabytecode, for example, into instructions that can be sent directly to a processor.
Just-in-time also refers to the hottest link in the business supply chain. The concept is to keep a company's inventory to an absolute minimum. Parts and raw materials are delivered by suppliers to manufacturers at the precise moment they're needed. In turn, manufacturers produce and deliver their products to their customers just-in-time to be sold. In this sense, just-in-time refers to just-in-time techniques, such as inventory control, supplier relationships, quality control, and the like. The shining example of this is Dell Computers.
Slang usage has morphed the meaning into something that can be handled or assimilated quickly, such as just-in-time training: small, easily digestible pieces of information.