Optional: (foo) means that this argument is optional. This means the command can be used without specifying it.
Required: [foo] means that you have to add this argument for the command to work.
Select: [foo/bar/baz] means that you have to select one of these arguments for the command to work.
@user means a mention of a user. You can mention a user by typing a @ followed by the users name.
#channel means a mention of a channel. You can mention a channel by typing a # followed by its name.
@role means a mention of a role. You can mention a role by typing a @ followed by its name. Before mentioning a role please make sure its mentionabel in Guild settings/Roles.