It specifies the actual filename of the SQL database. I have never used this parameter. I specify the server name and database name and let SQL server figure out where the file is stored.
I was a SQL db Administrator (among many other rthings) before I retired, and there were a couple of times when I had to move our database files from one disk to another. Any applications that used the attachdb parameter to connect would have failed. None of them did. I can't think of any reason you would want to use it either.