“A flash sale 一场快速的售卖” 的意思是 “限时特卖、限时减价”，即一种商家在短时间内减价出售商品的促销活动。这类活动的时间有限，但折扣力度通常较大。商家们会用 “a flash sale” 作为标语来吸引顾客。

例句
If you want to buy flight tickets, look out for the flash sales which are common after Christmas.
如果你想买机票，可以关注一下圣诞节后常有的限时抢购。

I was lucky – I saw a flash sale and managed to get this really cheaply!
幸运的是，我看到了一个限时抢购活动，所以买得非常便宜！

It would have been much cheaper yesterday in the flash sale – but it's finished now, sorry!
昨天限时大甩卖的时候，这件商品要比现在便宜得多。但促销活动已经结束了，很抱歉！