What does byte-compile a variable containing a lambda do?

[Lennart Borgman]

The help text for byte-compile says

    byte-compile is a compiled Lisp function in `bytecomp.el'.

    (byte-compile form)

    If form is a symbol, byte-compile its function definition.
    If form is a lambda or a macro, byte-compile it as a function.

I do not understand the last sentence. What happens? If a variable
contains a lambda and you do

    (byte-compile VAR-WITH-LAMBDA)

what happens? Is the content of the variable changed or does
byte-compile return something?