A ratio defines a relationship between two numbers compared to each other. For example if there are 2 girls and 4 boys in a classroom, the girl to boy ratio is 2:4. It is represented in fractions i.e 2/4.
Proportions tell you when the two ratios are equal to one another. For instance, 4:8 ratio would be proportional to 2:4, since both 4/8 and 2/4 are equal.
In the real world scenarios, ratios and proportions are very commonly employed.
NOTE: All Khan Academy content is available for free at www.khanacademy.org