On this page of the Undecember guide, you can read about titles. We will explain what they are and how you can get them.
How to acquire?
Titles are divided into four categories: Normal, Magic, Rare, and Legendary. In most cases, you will receive them after completing a given main quest or a specific act. In some cases, you will get them after completing an activity. Each of them has information on what you need to do to get it.
What do they offer?
Once you equip any title, it will be visible to you and other players. Moreover, each the titles has a unique effect that encourages you to use it. They boost individual statistics, such as resistance to specific elements, the damage you deal, etc.
You can see all the buffs you get from the titles by going to the Titles tab and then Owned Effect.