This is a module that loads images as they appear on the screen. It is widely used in combination with other modules.
When scrolling, the images loaded. So you can already see the first images, while others still have to be loaded.
As you can see below the offset is 100px. So the images are being loaded 100px before your visible page.
Blazy (actually be lazy) also shows when loading, a temporary icon ... This comes when you scroll too fast or on loading of the page.
It is a third party.