https://www.gnu.org/software/guile/manual/html_node/Compiler-Tower.html

Guile’s compiler is quite simple – its compilers, to put it more accurately. Guile defines a tower of languages, starting at Scheme and progressively simplifying down to languages that resemble the VM instruction set (see Instruction Set).

Each language knows how to compile to the next, so each step is simple and understandable. Furthermore, this set of languages is not hardcoded into Guile, so it is possible for the user to add new high-level languages, new passes, or even different compilation targets.

Let's use the tower metaphor for a moment. Consider Scheme:

Scheme
   |
   |
   v
Tree Intermediate Language (Tree-IL)
   |
   |
   v
Continuation-Passing Style (CPS)
   |
   |
   v
Bytecode

To compile Scheme to Bytecode, it's lowered through different stages until it's Bytecode.

We can also decompile, and raise code instead of lowering it - for example, here's how Guile-JavaScript works, which compiles Scheme to JavaScript, by first lowering Scheme to Tree-IL, then raising that Tree-IL to JavaScript through decompilation.

Scheme         JavaScript
  |                ^
  |                |
  v                |
Tree Intermediate Language (Tree-IL)

We take code through the tower to transform it into its desired form.

Hope this made sense. Or maybe it confused you even more =).