when I turned off my icloud photo library it said 'This iphone was not able to download full resolution photos and videos. if you continue the low -resolutions versions of these photos and videos will be removed'. what does this mean?

when I turned off my icloud photo library it said 'This iphone was not able to download full resolution photos and videos. if you continue the low -resolutions versions of these photos and videos will be removed'. what does this mean? Does it mean if i allow it to do so that it will delete my photos on my device as well? Or does it mean for only those photos that have been changed to full resolution in particular?