What is concave quads and how do I avoid them?
avoid making them…
a concave quad has verticies that lie inside it, or the opposite side’s vertices are swapped…
Basically it means that if you filled in the space between 4 points, there would be a “fold” across the middle. Usually you just need to nudge a couple of points around a bit then try filling again. Look from a couple of different views to see how the points align with each other.
Oddly, once you’ve filled in the face, Blender will then let you put the points back where they were - making a concave quad that it happily lives with 
Avoiding them is probably a matter of habit and experience (I don’t know coz I’m too new - but I get less conc-quads now than I did two months ago). Box modelling or modelling by extrusion of edges and faces avoids them but if you draw separate edges or delete faces then move vertices around without care and then try to make faces - you will often get the concave quad error.