When you walk around, things seem to be fairly normal. Gravity holds everything down so things are floating in the air. When you see a driver, no matter how crazy, careening down the road, you can say "Hey, this idiot is driving around 90 mph and is going through that red light". If you had a radar gun, you could be even more precise at getting the car's position and momentum.
However, what we think of as "normal" is not an accurate representation of the universe. Quantum mechanics exists to explain all of the absurdity that happens when you start looking at things on a microscopic level, like electrons and photons. For example, the idiot driving in the car that we mentioned in the previous paragraph. If we squeezed the driver down to a microscopic side, if you knew the exact momentum of the car, it would be impossible to get its exact position. Similarly, if you know the position, you can't measure the exact momentum.
For my quantum mechanics fun, check out the video below. If you can't see it, click here.