You can use your in-game Cash to upgrade your fighter's gear and give them better chances to win against stronger opponents. There are several ways to do this.
First, you can upgrade the gear items you already have. Choose an item you want to upgrade and 'infuse' other items you have into it. Once you've infused enough items, you can make an in-game Cash payment and press the 'Infuse' button. The chosen item will be upgraded and get higher stats.
Another option is to create a new gear item. To do so, you need to choose the item's type (weapon, armor, etc) and the desired rarity. Then you fill your Fuser with the items you already have. The number of items you have to feed into the Fuser differs, depending on their rarity and level. When the Fuser is full, you can press the 'Create' button and get the new item for an in-game Cash payment. Please note that this feature will be available no earlier than Q2 2019.
The higher the League you are in, the more effective items you can create.
Another thing that gives your fighter advantage over their opponents is sets. A set is a group of two or more gear items with the same prefix. For instance, Vortex Helmet, Vortex Gloves, and Vortex Pistol all belong to the Vortex Set. Every Set is designed to enhance certain abilities of your fighter (e.g. the Vortex set enhances damage).
When you equip your fighter with two or more items from the same set, regardless of their rarity level, you activate a Set Bonus. However, the more Rare, Epic, or Legendary items from the same set your character is equipped with, the bigger the bonus is. You get the biggest Set Bonus if your fighter is equipped with the full set of eight gear items, all of them Legendary. Every Set enhances certain abilities of your fighter (e.g. the Vortex set bonus enhances damage).
Sometimes the items you get may also have a Synergy bonus. It means that the item will provide a boost only if you equip a certain type of fighter with it. For example, you may get a Helmet which provides additional hit points only if a Protector fighter wears it.