#+foo A B

This says if the Lisp supports “foo” as a “feature” (a precise term in Common Lisp), the A will be included. Otherwise A will be treated as a comment. B will be included regardless. It’s roughly equivalent to

    #ifdef foo
    A
    #endif
    B

The opposite is #-.

    #-unix A B

is roughly

    #ifndef unix 
    A
    #endif
    B

So you can write portable source code this way.

    (defvar path-separator
      #+unix "/"
      #+windows "\\"
      #-(or unix windows) (error "idk"))

This will define a variable called PATH-SEPARATOR which is / for UNIX, \ for Windows, and a compile-time error otherwise.

The rabbit hole goes deep on this. There’s a reason it uses ‘#’, and as you pointed out, it has to do with built-in (dispatching) reader macros.