This is simply a reminder saved as a wallpaper.
To remove it you will need to change your desktop background (wallpaper).
Select the Start button, then select Settings > Personalization to choose a picture worthy of gracing your desktop background, and to change the accent color for Start, the taskbar, and other items. The preview window gives you a sneak peek of your changes as you make them.
In Background, select a picture or solid color, or create a slideshow of pictures.