This is a program for the Apple II series of computers running ProDOS. It displays all the "online volumes" which means that it displays all the disks that are currently connected to the system, along with their slot and drive numbers. ProDOS provides no built-in way to determine this information from inside the BASIC.SYSTEM command line. Here is an example of the output:

]-ONLINE
S3,D2 /RAM
S7,D1 /SYSTEM
S7,D2 /VISICALC
S6,D1 (NOT A PRODOS DISK)
S6,D2 /ONLINE