The Verge reports that Microsoft describes the Andromeda as "a new pocketable Surface device form factor." While actual details are thin on the ground, the device has been rumored to rely on a notebook app that will connect to OneNote and have a page interface that works like an analog notebook. You may also be able to run apps like Edge and Photos, and it may have built-in cellular support. Of course, as The Verge notes, the project is still a work in progress and could be canceled at any time. Case in point: the already developed and then killed Surface Mini.
If you're wondering what the new device might look like, The Verge points to this tweet: