Proportionality in math describes the relationship existing in two or more sets of numbers. For example, 5:10:: 10:20. In this proportion, the relationship between 5 and 10 is double and the relationship between 10 and 20 is double.
In law, the only example I can think of that uses proportionality is the situation in which two or more parties are liable for damages to another. For example, in a three-car accident, it is possible to assign "blame" in a proportionate way, making one defendant 70% liable and the other 30% liable. So, if the damages were $10,000, the first defendant would have to pay $7,000 and the second would have to pay $3,000.
We use proportionality in everyday life as well, in the form of analogies. We use these to try to see relationships between two ideas or events. An example of an analogy would be: Cats are to kittens as dogs are to puppies. This kind of thinking often allows us to try to make good predictions or decisions. If we do something and it results in a negative outcome, we think that if we do something similar, it could result in a similarly negative outcome. Of course, this is not a perfect way to make decisions and predictions, but it can be useful if the "things" or ideas are similar.