What do the lines behind icons mean? How to remove?

On a LG G5, I see that the icons of apps are often shown with some added lined symbols behind it, which are not part of the icons design itself. There are squares, triangles, variants of mirrored or rotated Z, H, and others: I'd be curious of it's meaning. Is that just on my device, and i must have made an unintended theme change somehow? **If not - how to get rid of the lines!**