These are my first pass galleys. I will get a second pass set as well. Once you see the size/font of your actual book, it makes reading a little different. You tend to catch things at this stage that you'd miss otherwise. You can see how long paragraphs will run in actual book form, how dialogue looks, etc...
(Here's a pic at the layout -- I sized it down, so that y'all can't read the page. I know, I'm mean...but no sneak peaks just yet!)
But the goal here is to check for grammar, typos, inconsistencies, anything that was missed in the copy edit. Galleys kind of drive home how important those revisions and copy edits are. Copy edits are your chance to dig in and fix, to rewrite if necessary. Once galleys arrive, your opportunity to tear things apart and rewrite is pretty much gone. (Unless, of course, you catch some gigantic plot hole that needs addressing). But, at this stage, rewriting in galleys will cause your editor a lot of extra work. Not good.
So how do I fix any mistakes I find in the galleys? Just like I did in the copy edits. Mark them with a pencil or pen, or some folks like to create a new document and just write a list of page numbers and the mistakes and then turn that in. Different strokes, for different folks. My editor is cool with however I want to do it, which is awesome. It's easier for me to just mark the actual page and send whatever pages I marked back to Pocket.
So there ya go. A look into the galley process from my end. :-)