If you’ve ever been on your final approach during landing and noticed the plane shifting around laterally, you’ve experienced cross-winds. Usually this phenomenon is mitigated by proper runway placement, which is why you’ll usually see runways going in different directions at larger airports. But if the winds shift or if the airport only has one runway, pilots will occasionally be forced to land in a heavy crosswind.
Kent can tell you more, but to cut to the chase, wind blowing on the side of the airplane during landing is not an ideal environment. It makes it really difficult to land. Check out this video of an A320 landing in heavy cross-winds in Hamburg. Scary huh?