Horizon Zero Dawn is an action-RPG through-and-through, complete with experience points and character levels. Each level will raise Aloy's base stats slightly, as well as providing more Skill Points. Those Skill Points can be used on her tech tree to increase her ability to hold her own in combat, sneak around the battlefield, and more. This article will explain how to earn experience and the best ways to do it quickly.
Almost everything you do in Horizon Zero Dawn will net you some kind of experience, be it hunting animals for a trifling amount, or completing larger quest-lines for heavy experience. Main storyline quests and some of the larger sidequests will grant the largest EXP bonuses, and many also come with a bonus Skill Point on top of the one you'll earn if you level up. Those can be time-consuming, though, especially in the case of quests that require you to journey to a new area of the map without a nearby fast travel point.
To rack up experience without quest-lines, try targeting larger machines. Once you've found a spawn location for robots like the Stormbird, Behemoth, or Thunderjaw, they'll appear on your map for easy location. You can go back and challenge them repeatedly, and once you find a good rhythm for taking them down, these are easily repeatable.
Main story quests and higher-level side-quests dole out large amounts of experience, usually with several steps that each grant their own EXP bonuses. Some specialized side-quests are also very high in value, such as completing the Cauldrons, destroying Bandit Camps, and completing Hunting Grounds challenges. When exploring a new area be sure to check every green "!" symbol you see on the mini-map, so that you have a queue full of quests to complete at any given time.
If you don't mind slower progress and want to be all Aloy can be, take your time riding from place to place during quests and taking down packs of enemy machines as you see them. Getting in the habit of doing this will net you the cumulative experience from beating down machines, as well as the quest experience.
Be sure to check out the full Horizon Zero Dawn guide for more on how to spend your newfound Skill Points, how to override new machines by finding all the Cauldrons, and more.