What is the context in which you heard that term? I've heard of "precompiling" before, but it's not a common concept in Java.
I remember this from Microsoft Visual C++. Compiling C++ code can take a long time. To make the compilation process quicker, Visual C++ would precompile C++ header files for you - so it would compile parts of the program in advance, so that everytime you change something, it would not have to compile everything from scratch.
Maybe some IDEs for Java do a similar kind of thing, and that's where you heard of it?