First Kill Rewards gives you loot every time you kill a type of monster for the first time, increasing in quality and quantity of drops the more monsters you have killed. These rewards should give you useful rewards based on what tier you are on.

Monsters

A monster is any mob that is included in the Monster Hunter achievement, along with the Warden which actually isn't in the achievement.
You can use the command /trigger get_first_kill_list to view which monsters you haven't killed yet.

Tiers

There are 6 tiers of rewards you can get. Where each tier starts and ends is found in the changelog, since they can change if new mobs are added.

These tiers go from giving you bread, coal, iron ingots, etc to giving you enchanted golden apples, diamond blocks, netherite ingots, etc.