images URL generation

Hi, i realise that retina 2x plugin is workink in saboresymomentos.es but the url where are shown the image it seems not to be in our host?
Appears like this:
https://i0.wp.com/saboresymomentos.es/wp-content/uploads/2017/06/bundt1Retina.jpg?resize=680%2C900

i don’t know what or where i0wp.com is??

somebody can explain me about?
thanks!