For those of you who are just now catching up, here's a brief history of the previous post-modern eras and how they relate to Dawn of DC:

The first era, New-52, followed an event called Flashpoint. In Flashpoint, Barry Allen, also known as The Flash, attempted to change the timeline to prevent his parents from ever dying. However, this caused a ripple effect through reality causing the world to reset. This reset created a new origin for the Justice League and overall shifted the tone to be more moody and dark. This era ended with Convergence which leads us into Rebirth.

Similar to New-52, Rebirth followed a major event called Convergence. Rebirth, once again, reset the world but also introduced the omniverse; the multiverse of multiverses. This event brought back some of the brighter and uplifting elements from pre-Flashpoint. It also brought back heroes like Wally West and introduced new characters like Superman's son, Jonathan Kent. This era ended with Dark Knights: Death Metal. 

Next we have Infinite Frontier, where the ominiverse exists and everything is canon. The main thing to note about this era is it begins with a passing of the torch for multiple characters; Jon Kent becomes Superman, Yara Flor becomes Wonder Girl, Wally becomes the main Flash, Jackson Hyde becomes Aquaman, and Jace Fox becomes Batman of New York City. Also the Justice League dies. All of this eventually leads to the Dark Crisis event.

Dark Crisis on Infinite Earths follows the death of the Justice League. The gist of it is: Jon Kent forms a temporary new Justice League to help combat the growing chaos caused by the Great Darkness spreading across the multiverse. We find out the Justice League isn't actually dead (obviously), Pariah, who was working with the Great Darkness, is defeated along with Deathstroke who was leading an army across the earth. 

Alongside Dark Crisis, we also have Flashpoint Beyond, a sequel to the previous Flashpoint event that lead to the New-52. Basically, Thomas Wayne - the Batman of Flashpoint - wants to replace that universe with another universe where his son, Bruce, didn't die. This all ends with the return of the JSA. 

Now we're all caught up to enter the Dawn of DC. Like all of the previous eras, the continuity continues but the world gets a light reset. Major things to note: the Justice League is disbanded and the Titans are back as the new defenders of Earth. Also, Amanda Waller wants to eradicate all metahumans. 

Usually with new eras, all of the titles are relaunched with new number ones, but this time, Dawn of DC will instead have a slow rollout of new titles throughout the year as well as merge pre-existing titles into this new era.