Stickers are ready made vector shapes you can insert in your thumbnails. The advantage of using stickers over images is that stickers can scale to any size without losing quality.
Adding lines, boxes, etc
Since stickers can scale to any width or height you can use stickers for creating lines, boxes, geometric backgrounds, etc
Stickers as backgrounds
It is also possible to use certain shapes to create basic geometric backgrounds to use with text or image as shown in the example below:
You can change the color of stickers by adding a color overlay.
First click on the sticker to select it, then use the color overlay drop-down to change its color.