Not a silly question at all. A member is a means of grouping records in the file together.
Most physical files on the AS/400 have a single member, but they can have others added using the ADDPFM (Add Physical File Member) command. Source files are prime examples of multi-member files.
Some applications will create a member in a temporary file for each individual user to keep their records separate during processing. You can use the OVRDBF command to process a particular member when the file is opened.
All the best
Some additional but potentially important info —
Physical files can also have zero members. IBM supplies a number of such files for use in various systems management tasks. The point of these is to supply us with the “file description” without actually having any data. You can copy these ‘model’ files into your work libraries and put your own data into them. An example would be a file that can hold (and describe) a set of entries from the system audit journal — AF entries, for example; audit entries for any Authority Failures that have happened. Physical files with no members are useful because of their descriptive attributes.
Also, actual SQL TABLEs cannot have more than a single member. Although you can use SQL to process native physical files and you can process SQL TABLEs with native I/O operations, you cannot perform native operations on actual SQL TABLEs that affect the number of members. The SQL Standard has no “member” concepts. (SQL can be directed to operate on particular members, but SQL itself can’t choose which member.)