A blank slate is a view would normally show a lot of information to a user—a list of projects, albums, tasks, and so on—but because the user is new, they haven't yet created anything.

The default behavior of many apps is to simply show an empty view where the content would be. For a new user, this is a pretty poor experience and a massive missed opportunity for you to give them some extra orientation and guidance.

A blank slate is usually some helpful text, hints and maybe a friendly graphic or icon. Now, because these views can appear on a per-feature basis, it's easy to be very task-oriented in the advice you give. If the user views the to-do list, you can give advice on making the first to-do item.

On a profile, you can give the user guidance to include a bio or add an avatar pictur.

The blank slate is only shown once (before the user has generated any content), so it's an ideal way of orienting people to the functions of your product, while getting out of the way of more established users who will hopefully "know the ropes" a little better. For that reason, it should be considered mandatory for UX designers to offer users a useful blank slate.