The FTP Manager option in your Cpanel hosting account control panel allow setting up and managing additional FTP accounts in your hosting account, there is an option to manage current FTP sessions as well. FTP are generally used for transfering files between computers. By using a FTP client, you can be connected to lots of public FTP servers with lots of publicly available information, of course there are private FTP servers that only allow authenticated users to gain access to the information stored within.
There are lots of FTP software packages available, some required paid license fee to use, some release under open source license and there are some in-built into web browsers (such as Internet Explorer), while some others are built into web applications (File Manager in CPanel is one example). Generally speaking, it is handy to pickup and master a FTP client for daily work, as most of the time you will need to upload/download files to/from your hosting account, it will save you a lot of time to have one FTP client you are familiar with.
CPanel allows setting up additional FTP accounts for accessing a dedicated and restricted folder in your hosting account, this is useful if you are setting up a team to work on web project, you can give each memeber a private area to work with and only the master user of the hosting account get's full access to all the content. If you are offering files to public, you can also set up an anonymous FTP account (i.e. guest login that requires no password to access) so that anyone can access a public area of your hosting account. Other than setting up private and public FTP accounts, you can also manage currently active FTP sessions, drop the session if it has been connected for too long.