Hi,

It's the order numbers of posts on the "Popular Posts" widgets. Because you didn't set a featured images for 4 posts above, so it doesn't display on those 4 posts - you can see an example how it display here: http://prntscr.com/vsj8d3

To hide it, please go to Appearance > Widgets > on "Footer Column #1" > edit widget "Soledad Popular Posts" there > and check to "Hide Order Numbers?:"

Best Regards,

PenciDesign