In Common Lisp, you get to decide which package each individual token of source code contributes to.
If a package abc exists, we can write abc::def. Now the abc::def symbol exists, if it did not before: the symbol named "DEF" in the package named "ABC". If we write (defun abc::def ...) we are binding that symbol to a function. Informally speaking, we are defining a function def in pakage abc.
If a package abc exists, we can write abc::def. Now the abc::def symbol exists, if it did not before: the symbol named "DEF" in the package named "ABC". If we write (defun abc::def ...) we are binding that symbol to a function. Informally speaking, we are defining a function def in pakage abc.