What does mean the status INVALID for an object because I
still can use a package which is INVALID ???

Answer 1:

An invalid status of an object shows that the object has to be
recompiled before next run.

This recompilation might have been neccessitated by some schema
dependecies or revoking any privliges from the owner of object.

Answer 2:

Invalid objects should automatically recompile on use.  If you are able
to successfully execute against an invalid object then if you check its
status immediately after use it should no longer be invalid.