Guidelines help you place the cropping box more accurately.
Show Guidelines on Image Tiles - Toggle lines on or off over thumbnails in the image list on the left-hand panel.
Show Guidelines on Image Preview - Toggle lines on or off over the larger image in the center panel.
Dark Grid Lines - Toggle between black and white line colors. This can be handy when working with predominantly black or white pictures.
Hide Top/Side/Bottom Guidelines - Toggle the top, side, and bottom composition guides on or off individually.
Green/Red/Yellow - Toggle the colored side guides on or off.
Top T/Bottom T - Adjust the top and bottom positions of the guide T-marks.