Welcome to LQ!!
"source" usually means "source code"---i.e. the original (readable) code used to compile a SW program.
In a typical Linux system, you need all of the source code--including headers--to compile the kernel. To compile application programs, you typically only need the headers.
Please tell us more about the context of your question---eg why you need to install user-mode linux, who told you to get the source tree, etc.