My single favorite thing (and I think I've mentioned it before) is something iserith wrote a few years ago and I found it very useful in my early days of getting a grasp on 5e rules. And that is iserith's posts on Adjudicating Actions. I went so far as to put it into Word, edit it, make a pdf of it, and give it to new players. My edited version is 14 pages long and it's basically an entire adventure done with nothing but Ability checks (or no checks at all if the DM rules auto success or failure).
The Combat portion of the game has lots of rules. But the Exploration and Social portions of the game can be quite tricky to adjudicate in a fun, exciting, and engaging way as the rules are sparse (relative to combat) and require a reasonable bit of extrapolation to get to a point the DM is comfortable with.