Imagine multiple stations transmitting at the same time and on the same frequency trying to contact a "rare" long distance station (DX). This creates a “pileup” of signals producing QRM (noise) as each operator attempts to be called back by the DX station, who picks out one call sign—maybe the loudest or the clearest or the luckiest—and makes a short contact, most likely a simple exchange of signal reports. Then the pileup starts again. Source ON All Bands.
Image credit Sharkmob.com
Pileups can be huge random events, and if you keep listening and carefully calling, you'll likely get through. It may be on the first or second call, or it could take a half hour of calling. Don't get discouraged. And remember, despite your best efforts, some DX will get away. Propagation will change, or the station will switch bands or modes, or may even QRT. You have no control over that, so don't worry about it. Importantly - don't let it affect your performance.
Careful listening makes a big difference. It will tell you where to transmit, and when.
Source HK3C
