Fair use is a doctrine that allows for limited use of copyrighted material without permission from the copyright holder. For example, it may be fair use to download a small portion of a copyrighted song for the purpose of research or education. However, it is not fair use to download an entire copyrighted song for personal use.
Another exception to the general rule against downloading copyrighted material without permission is the case of orphan works. Orphan works are copyrighted works whose copyright holder cannot be found. In some cases, it may be possible to download orphan works without permission from the copyright holder.
However, it is important to note that the law surrounding downloading copyrighted material is complex and constantly changing. It is always best to consult with a legal expert to determine whether it is legal to download a particular song from mp3skull.