This is a new feature in Version 2 - Line-us is telling you that there’s a drawing waiting to be drawn. Press the button and it will draw it for you - there’s a new drawing every day.
By default Line-us is subscribed to our daily drawing feed - if you click on the ‘Follow’ button in the App you can make it follow any sketchbook - one of yours, one someone’s shared with you or a Public one. You can also turn it off completely by clicking the un-follow button.