You can easily change the name and the description of your Moov. A description is not mandatory, but might be interesting if you share a moov with someone else, as that person will also be able to read that description. Additionally, when you publish a moov to the Moovly User Gallery, its title and description are published as well.
To change the name and description of moov, first go to the Details view. Below the video player, you'll see the current title (and description, if there is one) of your moov. Click the pen icon next to the title to edit it:

Change video title details page

Clicking that icon changes the view to 2 fields: one for the title and one for the description. Enter your text and click the Save button.

Change video description details page