FSSpec is a structure used to refer to a file on disk. In this respect it works much like a path.
The modern replacement for FSSpec is FSRef. Unlike FSSpec, FSRef cannot refer to files which do not already exist. Old APIs which took an FSSpec to specify a file which may not exist are usually replaced with an API which takes an FSRef to the paret folder, plus a name of a file inside it. This FSRef/name combination provides equivalent capabilities while avoiding the problems of FSSpec
One very common use of FSSpec