The Short Reads section has not been announced by Amazon, but it went live in the Kindle Store some time in late March 2014. It was first noticed by authors on KBoards, and it contains stories which are anywhere from 1 to 100 pages in length, with prices between free and $1.99.
It's not clear whether the new section has content that can't be found in other parts of the Kindle Store, but it does have a unique organizational scheme. The stories are broken up in to 6 sub-sections based on the estimated reading time (15/30/45/60/90/120 minutes). It's also not clear how many titles are available; based on the menu on the left there seems to be around 600,000 titles, all sorted by length.
The ebooks are a mix of fiction and non-fiction. I don't think I saw any excerpts, but there were a number of stories that had previously been published in anthologies, magazines, etc. A few minutes browsing also showed me that Amazon is showing content from several of their publishing imprints, including Kindle Worlds, Story Front, Kindle Singles, and Day One Journal.
All of these imprints publish short works, and their presence here suggests that Amazon launched Short Reads in order to make it easier for readers who simply want a short piece to quickly browse all the short works Amazon sells on a single page.
Starting with Kindle Singles (announced in January 2011), the 4 imprints were launched over the course of 3 years and cover a diverse selection of content. Kindle Worlds, for example, launched in June 2013 with a focus on licensed fiction. DayOne is a digital literary journal which launched in October 2013, and StoryFront launched in December 2013 as Amazon's fourth foray into short fiction.