You should always try to reduce a fraction until it can't be reduced anymore. To do this, look at your fraction and figure out the greatest number BOTH the numerator and denominator can be divided by. This is called the GCF - the greatest common factor. In the video, the problem you must solve is (15/18). What number can be divided into both 15 and 18 evenly? The answer is 3.
Divide your GCF into the numerator to reduce it. In this problem, that means dividing 15 by 3 to get a value of 5, our new numerator. Then do the same with the denominator. Here, it's 18 divided by 3 for a value of 6. Our new answer is 5/6. Can it be reduced any further? Nope. That's it! You're done!