3How to craft it
AnvilSet bonus
The bonus applies while the helmet, chestpiece and leggings are all worn together.
where it fits
Starter armor, and it knows it. You make this because you are already mining tin for a pickaxe and an axe, so the ore is close to free, and it keeps the first few nights and the first trip underground from killing you outright. The next ore tier replaces it quickly, so do not spend a long session hunting extra bars just to finish the look.
wear all three
The complete outfit protects you a little better than the three pieces would suggest on their own, which is worth remembering when you are tempted to keep some hat you found. This early, every point of protection is a real fraction of what you have, and a mixed outfit throws the bonus away for nothing.
tin or copper
Your world generates one of the two, never both. Copper armor fills exactly the same rung of progression, so a run in one plays out much like a run in the other. The helmet sprite is the giveaway if you are looking: the tin one has pigtails where the copper one has a bun.