Borderlands 3: How To Equip Class Mods
Even after choosing your Vault Hunter in Borderlands 3, you'll have so many other decisions to make down the line. One of the biggest overarching strategies you need to plan is how your Vault Hunter will grow. An important part of that process involves finding, understanding, and using Class Mods.
Class Mods provide your character with buffs that often synergize with their abilities. More importantly, they give extra skill points to specific skills, and these can go beyond the cap. For example, Amara has a skill called Mindfulness, which gives her improved movement speed and shield-regeneration delay for every hit she takes. Normally, this skill has three ranks. If you find a mod that adds more Mindfulness points, you can go beyond those three ranks, improving its usefulness even more.
You'll likely find Class Mods in loot drops as early as Level 10. However, you won't be able to equip them right away. The slot for it in your inventory will be gray and unselectable. Unfortunately, there's nothing you'll be able to do about it unless you start rushing through the main story.
On Promethea, one of the earlier missions you get will be called "Hostile Takeover." Without spoiling too much, your goal is to liberate the planet from Maliwan's control. After you beat the boss called Gigamind, you'll unlock the ability to equip Class Mods.
As long as the Class Mod is for your specific Vault Hunter, you can equip it. If you ever feel the need to tweak your skill trees to benefit more from your mod, you can always re-spec at a Quick Change Station for a nominal fee.