With Follow-Up Mode, you can ask Alexa for additional information or to carry out further actions after your initial request. So, when you say something like "Alexa, show me comedy movies" and it displays a selection of such films on your TV, you can ask it to play a certain flick without having to say "Alexa" again. You might also use Follow-Up Mode to ask for extra info about the weather or items on a list. You'll need to switch on the feature from the settings in the Alexa app if you'd like to use it.
Multiroom audio is again fairly self-explanatory -- ask Alexa to play a song, playlist or genre "everywhere" (including your home theater setup) or on certain groups of devices, and it will do just that. The feature supports Amazon Music, iHeartRadio, Pandora and other streaming services. There may be some lag compared with your other devices, but you should be able to rectify that through the TV Cube's settings. An update that includes both features will start rolling out today.