What is NetGalley?
NetGalley is an ARC (Advanced Reader Copy) distribution site used by major and independent publishers alike to make advanced copies available to vetted and dedicated reviewers. In brief, how it works is that you list your book on NetGalley for however long you choose, and reviewers can decide if they’d like to review it. From there NetGalley handles things. It’s a lot more complicated than that, but this is the simple description. Books can be not-yet-published, newly published, or even published sometime in the past. Here’s an article with multiple links that explains it in much more detail (especially Susan K. Quinn's posts).
The main issue for indies is that it costs hundreds of dollars for a NetGalley account under which to list your books (which makes sense for large publishers who are putting out a dozen or more books a year. For an individual, it may be less cost effective to have your own account). To help make this feasible, indies and private organizations have started co-ops that allow nonaffiliated authors to “rent” a slot for a month or more through the co-op, and then have their book listed under that co-op’s NetGalley account. This is a very effective way of getting independently published books in front of readers without a huge investment.
List of a few indie co-ops (there are most likely many more; you may find them through a Google search or by asking other authors you know):
Broad Universe, open to anyone
Patchwork Press, open to anyone
Pikko’s House, open to anyone
Open to SFWA members