Positioning

What defines positioning?
Knowing where to put yourself at any moment to use your surroundings to your advantage defines good positioning.

This can mean positioning close inwards to land to knock people off, using staircases to get low ground advantage and hold combos, using walls to start combos, and going around pillars to get the first hit.

Here's an example:
Some specific examples:
Being near a wall - When you put your back to a wall, it allows you to hit your opponent without taking knockback. This will allow you to stay in range of your opponent while on damage cooldown, allowing you to easily start a combo.

Staircases - When fighting on staircases you can use low ground to your advantage, and quickly run + jump up the stairs to change positions to throw off your opponent's aim. If you combo your opponent while above them on the staircase, you can hold the combo while running down the stairs.

Void - When you are about to fight players, you can begin angling away from the void to try to knock them into the void. On the other hand, good players will try to position away from the void, making it easy to predict their movement, allowing you to catch them with projectiles (skywars) or blocks (in bedwars).

Headhitters - When there are headhitters (ceilings), you can use them to give yourself a momentum boost (by spamming jump while sprinting under them), and you can use/create them to make yourself take less knockback.

Pillars and corners of walls - When you have no other option, you can use these to try to throw off your opponent. (Most used in bedwars and wool wars.) You can run around the corner, then shift to make your opponent lose your position for a moment. Then, just as they are about to round the corner, you can run up to them and more likely get the first hit
This usually works because of a combination of ping causing a delay between when they see you and when you hit them, and also because when players need to turn corners, their momentum will decrease slightly because they aren't moving in a straight line. Since you are coming at them straight, your momentum will be slightly greater than theirs, increasing your chances of getting the first hit (but not guarenteed).
Creating Positions
Lastly, you can create your own places and environment to take advantage of. With blocks, you can make walls, pillars, headhitters, and platforms to use against your opponent. The thing most good pvpers underestimate is positioning, and it can often be the one thing good pvpers and high cps players will be lacking.