Perspective correction
Perspective correction straightens converging lines, like buildings that seem to lean back because you shot upward. The software rotates and stretches your image so verticals stand straight again. It's essential for architecture and interior photos.
In Lightroom you use the transform panel (Upright) for this: the auto setting often fixes a lot in one click. If that doesn't work well, use guided mode to draw two or four lines along edges that should be straight. Keep in mind that correction crops your image, so leave some room around your subject when shooting. Too strong a correction can stretch subjects unnaturally, so find the balance.
Read also the in-depth explanation: Perspective correction explained.
Related terms
Prefer a look in one click?
The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.