Click on the triangle to see the cropping warning:
A large percentage of the picture is being cropped. Click on the picture to inspect or adjust the cropping:
In the illustration above, the sides of the picture will be cropped off.
If you choose Uncrop, you will print the entire picture, but you may get white space on either side of the photo: this happens when you print a long photo on a rectangular paper.
You can also click-and-hold in the center of the cropping area to move it right or left, and you can grab a corner point to change the size of the cropping rectangle.