In spite of our best efforts when scanning or taking a photo, the verticals aren’t always vertical or the horizon’s not horizontal. An image might only need a minor adjustment, or it may need a significant rotation.
To make that happen, go to Adjust/Straighten Image and a dialogue box will appear showing your photo with a grid overlay on the left and some options on the right. The second of the two options concern the grid overlay. Make sure Grid is checked and that the color of the grid lines shows up enough on your particular image. Below that is a slider that changes the grid size from larger squares sliding to the left and smaller squares sliding to the right. You have to decide which size works best as a guide in straightening your image.
Alternatively, you could type numbers into the numeric box to the right, with plus numbers causing a clockwise rotation and negative numbers representing a counterclockwise rotation.