Hunting Grounds are special trials that can be found throughout the map that offer you a chance to test your skills using various weaponry and against Machines. There's a total of five Hunting Grounds in the game, with each one offering three different trials. The trials are graded like any normal competition is, with different named marks that simulate a gold/silver/bronze tier of rewards, respectively called Blazing Sun/Full Sun/Half Sun.
The Blazing Sun and Full Sun times are generally pretty low and require you to perform very well, while the Half Sun time is always 20 minutes. If you don't complete the trial within that 20 minutes or do something like kill all of a specific machine without fulfilling the requirements, then you will fail and need to restart the trial. All trials will give you experience for finishing them the first time, as well as a related box in your inventory that you can open for some goodies (usually Metal Shards and a crafting ingredient) for each time you finish. Complete all of the trials at a Hunting Ground with a Blazing Sun mark to receive a Skill Point.