Generally 90% of DnD stuff in game involves rolling a twenty sider and adding something to it. There's a target number you need to exceed to be successful.
To me, that's 20% of D&D.
What it's really about is roleplaying. That can mean a lot of things. But to me it generally means....
-Playing a character, with their own motivations specific to who they are. Rather than just playing yourself every time.
-Thinking about situations from your character's perspective. (What does my goodly paladin think about this versus my self-serving thief?)
-Learning to ask questions about your environment so you can make informed decisions. This is something I commonly see new roleplayers not understand.
-Formulating a plan or response to the situation you've been presented. If you're in a barfight, it behooves you to ask questions like "How many guys are there?", "Are any of them armed?", "Are any of them especially big?", "Where is the nearest exit?", "Do any of them look like a wizard?" Getting all the details of the scene and then working out a plan or strategy is what roleplaying is all about.
As an aside to the first point, lots of people coming to roleplaying games for the first time think it's like a video game. Where you're "you", wearing this skin and you're there to kill the monster and get the loot. And that does a big disservice to actual roleplay. Think of playing a character sort of like creating artificial restraints for yourself. If you're playing a self-serving thief who only cares about gold and winning, most situations will be approached the same way: to win. But if you're playing say a Lawful Good Paladin or Cleric, you've got a set of morals and principles which mean you approach different situations differently. So while you might slay that Goblin that jumps out of the bushes without a second thought, you might NOT slay a poor beggar with a stick who is just so desperate for food they're willing to waylay people on the street.
Mechanics are easy, in the end. It's just memorization and repetition until you internalize the rules. The real joy of roleplaying is actually
playing a character, having it interact with the GMs world and ending up with a narrative that came about because of the character you chose to play, and how you chose to play them.
For example, if you generally consider yourself a "nice" person in real life.....it can be a lot of fun to play a truly evil bastard because it requires you to think and act in a way that YOU, the player, would not normally act.
Most new players just focus on "min/maxing" so they can be as effective in combat as possible, because it's a simple goal to shoot for and it's what most people think of when they think roleplaying games. And they let the actual roleplaying happen if and when it will. But do that long enough and you eventually get bored, because you end up making the same character game after game, because your only goal is to win in combat. Eventually you realize the only dynamic thing you can really do in roleplaying games is roleplay different kinds of characters, because at the end of the day it produces more varied situations and narratives than "I hit him with my +5 Longsword/+5 Axe/+5 Spear/Fireball/Lightning Bolt/Power Word Kill/etc...."