What do you call this plus sign and where is it coming from? How can I remove it from Adobe Illustrator?

What do you call this plus sign and where is it coming from? How can I remove it from Adobe Illustrator?

Minor Problem. Which I don't understand. Please help me understand.
So at the center of one of my Artboards, there is this plus sign even if I'm not selecting anything, this is still showing. How can I remove it? Thank you in advance.