We are all observers. 

We observe what's going on around us, either consciously or unconsciously. Everything from how people behave with us, what the latest fashion style is, what the cool restaurants are, what features companies are rolling out, etc.

And we create theories from those observations. Theories like, "if that person was nice to me all of a sudden today, maybe it's because they want something", "if the person heading Apple's new devices division is now reporting directly to Tim Cook, maybe they're about to release some new kind of device, like a car". 

This is how scientific theories are formulated too (most of the time), where we come up with theories to explain real-world phenomena. 

The most successful theories are the ones that make a prediction that can be tested. "If the theory of gravity is true, then any object will drop to the Earth in exactly the same amount of time if dropped from the same height, irrespective of their masses". And they stand true after the tests are performed. 

However, most theories that we come up with in our heads about the day-to-day things that we observe can't be scientifically tested. They can be tested nonetheless. 

The faster we put our theories to test, more accurate our view of the world.