A ratio can be written as a fraction. So 6:16 can be written as 6/16. A fraction can be reduced by dividing top and bottom by a common factor. 6/16 has a common factor 2, because 6 and 16 are both even numbers. Divide 6 by 2 and we get 3; divide 16 by 2 and we get 8, so the fraction becomes ⅜. Now we put this back as a ratio: 3:8.
A ratio is simply a comparison of two or more quantities. Imagine we have a bunch of fruit: 11 pieces of fruit. Three fruit are apples and the rest (8) are bananas. If we take two such bunches, we will have 22 pieces of fruit, consisting of 6 apples and 16 bananas. The number of apples and bananas has changed but the ratio stays the same. For every 3 apples we have 8 bananas. If we add more and more similar bunches, the quantities of fruit increase but the ratio stays the same. That’s why ratios are useful.