There is a detailed description in the app manager. Basically it acts as a two way baby monitor. You can set the audio alert threshold and the N900 will listen for sounds from the baby. If the baby cries and alerts the N900, it will call a preset number (the parents number) and then you can hear the baby and come see about him/her.

I can imagine it for other uses too, monitoring rooms or the house, even pets.

I havent tried it yet but I saw it in the app manager about an hour ago.