The value set for minimum firmware version is the lowest firmware version your channel will be available for install.
For example, if you select v3.1 (which I believe is all older Roku players) then your channel will be available on all Roku players that have version 3.1 or higher installed (3.1, 4.0, 5.0, and so on). If you select, for example, v5.1, then your channel will not be available on any firmware version older than 5.1, but will be available on all players that have firmware version 5.1 or newer installed.