More or less what it sounds like, dragons breath does damage to the player like 2-3 times a second, and usually around 3 hearts per hit. So its pretty much instant death, with very little chance of escaping the acid.
When compared to java, this is extremely broken, as the breath over there only does damage once every second, or second and a half. Even with that amount of delay, its still very effective at killing people
Another side effect of this massive amount of damage bug, is a huge amount of knockback. You can easily fly 20 blocks just from touching some of the breath, and its strong enough to launch you about 5-6 blocks into the air.