NetherRealm says Injustice 2’s Gear system, which lets you outfit your character with not only cosmetic items but also new skills that impacts gameplay, brings a “new level of depth to the game.” As you can see in the trailer embedded above, there is a lot of variety in terms of what you can choose.
For a much deeper dive into Injustice 2’s Gear System, including commentary from the game developers, you can read GameSpot’s preview from earlier this year. One concern some may have about the Gear System that newcomers might get dominated by those with leveled-up gear. NetherRealm has a solution for that–you can read about that and more here.
The Gear system is separate from Injustice 2’s microtransaction/currency system, Source Crystals. You can use these to purchase cosmetic items that do not impact gameplay.