If you want to perform some command on several groups, and they appear subsequently in the group buffer, you would normally just give a numerical prefix to the command. Most group commands will then do your bidding on those groups.
However, if the groups are not in sequential order, you can still perform a command on several groups. You simply mark the groups first with the process mark and then execute the command.
Also see Process/Prefix.
If you want to execute some command on all groups that have been marked
with the process mark, you can use the M-&
gnus-group-universal-argument) command. It will prompt you for
the command to be executed.