This page explains what Hunting Grounds are and how they work in Horizon Forbidden West.
Hunting Grounds have returned from the first game, which are special trials that have you perform specific tasks in a time trial. Each Trial will require you to do a different task and upon clearing them in time, you will be rewarded with some loot. Depending on how long you take to finish, you will earn different stripes, which are basically like medals. Quarter Stripes are equivalent to a bronze medal, Half Stripes is equal to a silver medal, and Full Stripes is similar to the gold medal. Each set of stripes will have a greater reward waiting for you, but the higher ones will require you to finish the Trial in a lesser time. You will also receive a Grand Stripes prize (Quarter, Half, and Full) for completing all of the Trials at a particular Hunting Ground.
For Trophy purposes, you only need to complete all of the Trials in the game with a Quarter Stripe ranking. You will unlock Obtained 3 Stripes at a Hunting Ground for earning at least a Quarter Stripe in all three trials at a single Hunting Ground, and Obtained 3 Stripes at All Hunting Grounds for getting at least a Quarter Stripe in all three trials at all Hunting Grounds. There is no reason to go after the Half/Full Stripes, if all you want to do is earn all of the game’s trophies. However, if you do decide to get Full Stripes in all of the Trials, then you will be rewarded with the Tinker’s Pride weapon, a legendary Tripcaster.
As you complete the various Trials in the Hunting Grounds, you will earn Hunting Medals, which can be traded in with Dukkah at the Maw of the Arena in the western portion of the map. This will allow you to purchase some very powerful weapons, so it’s a good idea to complete the Hunting Grounds to earn these medals. There is no entry fee for each of the Hunting Grounds and you are free to retry them to better your score, if you want. However, any resources spent during the Trials will stay spent, if you finish or quit via speaking with the Grounds Keeper. Should you die during the Trials, then all resources spent will be given back to you and you will have to begin from the start.