Basically, an extent is how much data DB2 writes or reads to disk at one time.
If prefetching is being used for a particular query (DB2 decides whether to use this based on how much data is needed from disk for that query) then multiple extents are requested at the same time, even before they are actually needed by DB2 (in anticipation that they will be needed very shortly).
An example of prefetch would be if a query is doing a table scan on a large table, DB2 knows that it will eventually need all the pages in memory, so it puts them into memory before the page is actually requested. All DB2 table and index I/O (except for LOB's) are done into or out of memory (bufferpools).
I would recommend that you look in the PDF versions of the manuals for more explanation. They are free to download.
M. A. Feldman
IBM Certified DBA on DB2 for Linux, UNIX, and Windows
IBM Certified DBA on DB2 for z/OS and OS/390