In Fates , there is a rock-paper-scissors system that allows certain weapon classes advantages over the other. If playing on Normal difficulty, one can easily ignore the entire weapon triangle as a system and do just fine, but if you would like to make things easier, or if you are playing on Hard or Lunatic, the weapon triangle absolutely must be obeyed! Being at an advantage can dramatically increase accuracy and being at a disadvantage can dramatically increase the enemy's accuracy, in addition to altering damage that can be taken or given!
When engaging the enemy, the combat forecast will tell you immediately if you are advantaged or disadvantaged. Weapon icons also have colors behind them, a color that takes up most of the box, and a sliver of color at the bottom of the square. The color of the square is super effective against the sliver of color you will notice, so if you forget, the game will remind you.
Red weapons are super effective against Green, which are super effective against Blue , which are super effective against Red .
Swords, or katana and magic, or scroll weapons will be super effective against axes, or clubs and bows, or yumi, which are super effective against spears, or naginata and daggers, or shuriken, which are super effective against swords, or katana and magic, or scrolls.
Characters who can equip multiple weapons have a small advantage over more specialized classes, although this does sometimes come at the cost of having lesser stats or inferior skills. When building a balanced team roster, make sure you cover all of the weapon classes so that you will always have an answer to your enemy.
Note: Some weapons exist that purposefully invert the Weapon Triangle, or are designed to be super effective against what they are usually weak to. And still other weapons are designed to be effective against specific classes of weapon regardless of the triangle, similar to how Beastslayer is effective against mounted units. Read all weapon descriptions carefully if you're suspicious.