She SSH'es into her father's machine from her Raspberri Pi. Presumably like any good hacker she used social engineering to obtain his password. She then uses the ps or top command to find the process ID of the "Sublime Text" editor he is using - the number she reads out. She then uses the OSX say command on the command line on his computer to make it speak to him:
$ say "Dad watch out"
Given his reaction it appears he doesn't consider the possibility that anybody has remote access to his machine and he also doesn't seem to know about the OSX say command, hypothesizing incorrectly that the kids have set up a timed MP3 file. Finally, she uses the kill command on his machine to kill the "Sublime Text" process, closing down the windows he is working on.
Ha ha ha, adorable!