Because of your (the photographer) position to the subject, and sometimes the lens you are using, the captured image can have its perspective skewed. You can fix this by making manual adjustments in the the Lens Correction panel in Lightroom 4 and 5 or the Transform tool in Lightroom 6 and CC.
Last spring Tammy recorded an awesome video, below, showing how to use the "Manual" feature in the Lens Correction panel (before the Transform tools were pulled out of the Lens Correction panel) to bring one side of an image closer, so that it matches the opposite side. I have used the Lens Correction/Transform panels many times to do similar things to photos, pushing away part of an image or pulling it forwards. The image above shows the Transform panel below the Lens Correction panel, as well as the various sliders you have available.
Reminder: Tammy shows using the Lens Correction panel. If you have Lightroom 6 or CC, you will use the Transform panel.