Defensive driving is driving with the assumption that anything can and might go wrong. It is taking that view that the roads are full of poor drivers, so you need to be extra cautious.
Think about someone living life on the edge. It would only take a slight mistake for them to fall off the edge. Someone who goes about things with more caution is more likely to survive a hazardous situation.
Assume the unexpected
You see a car indicating to pull over and decide to pass. How can you be sure that is their intention? What if they pushed the indicator the wrong way? Or put on their hazards, but the bulb on the other side is blown? If you wait a few seconds until they have pulled off the road before passing, you nullify a possible crash risk if your assumptions were wrong.
Know your escape route
Before you make any maneuver, figure out what you will do if something goes wrong. For example, before overtaking, decide what you would do if a speeding motorcyclist comes flying around the bend at you. Knowing your escape options before you make your move means you do not lose precious time wondering what to do if it should occur.
The faster you drive, the less time you leave yourself to react to a dangerous situation. Even a few miles per hour slower could mean the difference between seeing something and braking in time or hitting it.
You need to know what is happening in front of you, so it may seem logical to spend most of your time looking out the front windshield. Yet surprises can come from any direction and injure you. Scan between your mirrors and windshield, so you have an up-to-date picture of everything going on around you. For example, if you notice the driver behind you is impatient, you will not be surprised when they overtake with insufficient space and pull in the tiny gap in front of you, causing you to brake.
Defensive driving increases the chance you avoid the risks other drivers pose. Yet sometimes, even that is not enough to avoid a crash, and your only option is to learn how to claim compensation.