Often times when taking pictures with a wide angle lens, especially photos of buildings and interior of homes, lines in your pictures can be skewed. Many do not realize it at the time, but after it is pointed out you can become frustrated and find it difficult to get your lines straight. Photographers can use a tilt shift lens to take care of this problem, or it can be corrected with software.
In this example, the lines near the left side of the original image are slanted towards the right. It looks like the window or the entire of the left side is going to fall in. Now take a look at the corrected image and you will see that the lines on the left are no longer slanted.