A result is representative when the group of respondents have the same features than another larger group of people.
In order to understand if your results are representative, you need to define what your sample should be representative of : the population of a country, a subset of a population, target consumers of a product, etc.
Here are some rules to keep in mind :
A Brand Performance measure is representative of a country or is representative of a specific target that you have set up.
A Brand Lift measure is not representative of the population of a country. It measures the difference in response between two groups : people who have seen your ad, and people who have not seen your ad.