What this means in practice is two things: first, that writers can't pay to have their books featured on the site, unlike the case with many sites that will promote an author's book for a fee; secondly, The Midlist exercises a degree of quality control by choosing books using "a combination of literary merit, reader activity around the title, and social brand engagement around the author". In other words, books are chosen because The Midlist believes they are worthy of wider promotion because of the intrinsic quality of the work itself and the professionalism of the author.
The Midlist is a spin-off from Libboo, an enterprise that has been going for several years as an author-focused initiative. By creating The Midlist, the organisation has 'crossed to the other side,' so to speak, in its attempt to engage primarily with readers, offering them excellent quality literature through an email subscription.
It's interesting - and perhaps rare - to find a site that seems truly dedicated to helping both authors and readers. Good authors with best-selling potential can find a place to promote their work, while readers receive one email a day with only two promoted books on it, though from a variety of genres. It has the feel of a professional and dedicated site and should be warmly supported.
The home page for subscriptions is here: The Midlist.