Collection, why COW collection are important

[Gary Willoughby via Digitalmars-d]

On Saturday, 31 October 2015 at 23:02:12 UTC, deadalnix wrote:
> The hack team recently released project to add COW collection 
> to hack in addition to current, reference type, collections. 
> You can find explanation here :
>
> http://hhvm.com/blog/10649/improving-arrays-in-hack
>
> As collection are discussed here, I think this is relevant, 
> especially the explanations as of why COW matters.

What does COW mean here?