What do the number of reads and writes reported by the fn_virtualfilestats() system function mean?

Fn_virtualfilestats() can be a handy tool for understanding the amount of I/O that's happening within individual files on your SQL Server system. The function reports the data that Table 1 shows. The definitions for NumberReads and NumberWrites seem obvious, however, SQL Server often uses the term read to refer to reading a particular database page. Pages in SQL Server are 8KB, and SQL Server must read an entire extent—a group of 8 pages, 64KB total—to read a single page. The short answer to your question about what the value for NumberReads or NumberWrites measures is that the values are the number of read and write requests issued by SQL Server to the OS. You can't directly infer how much data was read or written for any single request.