You can create your own commands in your modlet using the
'"xyz" command is invoked' event type in the Event block
(from the Events tab). Often the command doesn't need any more
information, like '.play' in spleef. It builds the arena and starts
the game without any extra info.
Parameters
However, if you need more information, you might want people to enter
parameters after the command. Say you are making a command to
teleport the player. The player will want to specify the destination
coordinates, like this:
.tp 0 105 0
Your command can find the user-provided parameters in a list of
strings called 'parameters'.