We would hope to never get in a situation where you must fight for your life, but we shouldn't hope; rather, we should prepare. Many of your thoughts in a life threatening situation will be skewed due to adrenaline and panic. The best way to counteract this is by understanding the inner workings of self defense before an situation ever arises.
A common myth we are shown in the media, and even self taught from playing with toy guns as children, is that as long as you are behind an object you are safe. Apply this to a gunfight or active shooter scenario and you are likely to be injured or killed by a threat.
There is a great difference between cover and concealment and the way your safety is then affected.
Concealment is as it sounds. It provides visual cover to protect you, but it often offers little to no ballistic cover. Household walls, bushes, wooden fences, and other penetrable objects are simply concealment. The only form of safety concealment offers is the inability to be observed. Think about it as hiding in a game of hide and seek, but one that could save your life.
Cover is what will stop a projectile from coming into contact with you. Many objects that you would think are cover are sometimes not, so it is important to think about what provides ballistic protection. Cars are an example of poor cover. Many calibers of bullets can pass through the body of a vehicle with ease. Real cover will generally come in the form of thick concrete and other dense objects. Cover will function as concealment as well.
Deciding whether something is concealment or cover takes only common knowledge. Out in public there are often thick concrete support beams that will function as cover, but something such as a sheetrock wall or trashcan would be concealment since a projectile would pass through with ease. In the event of a gunfight or active shooter, your first goal should be finding cover. If cover is not an option, at least find concealment.
Next time you're out in public take the time to observe what is around you and judge it as concealment or cover. It may take a little bit of extra thinking, but one day it may be useful. You may even be surprised by what is actual cover and what you encounter that is merely concealment. Both play very important roles and can be helpful in varying situations.