Micro editing refers to (yep, you guessed it), the micro aspects of your writing. This part of the editing process happens towards the end of your work and focuses on the smaller details in your writing. There are many stages to editing, however they mostly fall into two sections; the micro edit and the macro edit.
When you start a micro edit, your writing should be completed. If you haven't finished writing or finished your macro edit, then the micro edit is pointless as you will most likely be making changes that make your micro edit redundant.
So if you're completing a micro edit on your own work or someone else's... Focus on: Language - Is the voice consistent? If you're writing the book in English, make sure there are no Americanisms popping up. Or if you're writing from third person, there are no 'i's' appearing. Repetition - only ever has an impact if you're using it affectively. If you have no reason for repetition, take it out, use another word. Make sure there is a wide range of vocabulary when editing and try to avoid using words closely together (within reason, of course). Redundancy - is this important? Is it contributing to the story? Does it work with the story. A lot of the time, an idea can change and that needs to be changed throughout the book too. Clarity - Is your writing clear, simple and concise? Check your sentences and make sure you're not putting too many subjects into one. That each sentence works with each other to Authenticity - Does this make sense? Is it possible? Would your character really do/say that? Continuity - Your character is wet from the rain, but earlier on you mentioned that it was sunny... The story is set in 2000s and your character has an iPhone? Show and tell - Make sure that in the writing, you're showing your readers what's going on. Avoid over-explaining what's happening as most likely they picked up on it through description and dialogue. Beginnings, endings and transitions - Is the book consistent? Especially in longer pieces, it's hard to carry consistent tone/pace throughout. Proofreading - This part is technically part of the micro edit, as it's completed in the final stages of editing. However, a proofread must not be done until there are no more changes to be made to the rest of your work, that all writing and editing is done. Otherwise, again, it will be redundant.
(The Artful Edit, Susan Bell)
I think the micro edit can always seem very daunting as it feels like there is a lot to consider. However, a lot of these points tend to blur into each other (for example, authenticity and continuity. And you could argue that repetition is part of language).
If I could summarise the micro edit into one sentence, I would say it's to make sure that your work runs smoothly and consistently.