Fileshare is a client/server file handler, supplied as part of Enterprise Server. When you configure an enterprise server to use Fileshare, you are making the enterprise server a Fileshare client, so that some or all file I/O requests coming from applications running in that enterprise server will be handled by a Fileshare server.
You can configure some or all files that are accessed from CICS applications to be accessed via Fileshare. You can also configure files accessed by JCL, and even the catalog itself, to be accessed via Fileshare, if you wish. However, the major benefit of Fileshare is when used with CICS.
Although its name implies file sharing, the primary reason for using Fileshare is to support transactional access to data files using the COMMIT and ROLLBACK verbs or EXEC CICS SYNCPOINT when using CICS. Without it, there is no transactional behavior in CICS. Fileshare also provides rollforward recovery to ensure the integrity of your files in the event of a system failure.