What are pileups?

Some articles on pileups:

The Big One (NASCAR)
... Though multi-car pileups have occurred at nearly every NASCAR track, the term is most commonly used to describe large multi-car Sprint Cup and ... at Daytona and Talladega are the most vulnerable to pileups ... preseason event which leads up to the Daytona 500) but their smaller fields mean less risk of pileups ...