At first, when I saw this schema, I thought; huh! what is he trying to tell us.
But as you may already know, in mesh topology quads are important. Because quads are easy to subdivide, creating more quads. And with quads you can work with loops. Adding more mesh detail on places where you need them.
Quads are faces with only 4 vertices. Making it easier to subdivide and still having quads.
But sometimes, you need to join 2 quads with 3 or more quads. And still having a quad topology.
The following links, shows you an schema how to join quads. Start at the top row (many quads), follow the color column down, for less quads.
Subdivision modelling step-down guide
Happy render