How fast you get real work done is an easy way to think about intensity 
Specifically, intensity is defined as power: force multiplied by distance, then divided by time. Simply put: Intensity is doing more work faster.
Why is it important
“Intensity is the independent variable most commonly associated with maximizing favorable adaptation to exercise,” CrossFit Founder and CEO Greg Glassman wrote in April 2007’s “Understanding CrossFit.”
Favorable adaptation includes improved body composition and improved health markers such as fasting glucose and triglycerides. It takes people from sick to well to fit.
I’m lost… Give me an example 

Crudely translated, it means this: Do more work in less time—not more work in more time. 

Explained via a CrossFit scenario, if you took 10 minutes to do Fran and then did another workout because “10 minutes wasn’t enough,” you did not perform Fran with intensity. If you had, you’d still be on your back. Likewise, you will not reap intensity’s benefits.

What if I don’t like doing workouts with intensity 

Mate! most people do not get excited about putting themselves through an intense workout… but it is where growth is, of both your mind and your body. 

You will be amazed at what you can accomplish and how good you will feel after

Intensity is impressive, hanging in the gym for 2 hours… not so much