Hide / Show Guides: Go to View in the menu and select Show and select Guides to toggle hide and show guides.
Why does my Photoshop have blue lines?
It's just a guide line pulled down (or across) from the ruler. As Derek said, it is a guide. It is only visible in Photoshop and will not appear on the printed or exported photo. Guides are useful for aligning elements in a picture.
How do I hide the ruler lines in Photoshop?
Show or hide a grid, guides, or Smart Guides
Choose View > Show > Grid. Choose View > Show > Guides. Choose View > Show > Smart Guides.