You can use audience targeting to show your ads to:
specific websites, based on specific interests
groups of websites, based on specific interests
specific groups of people, based on their interests
specific groups of people, based on their location
Adding audiences allows you to reach people based on their specific interests as they browse pages, apps, channels, videos, and content across YouTube and the Google Display Network as well as on YouTube search results. You can select from a wide range of categories — from autos and sports to travel and fashion — and we’ll show ads to people who are likely to be interested. Depending on your advertising goals and the stage of the purchase process your customers are in, you can choose to add different audiences to your ad groups.
Audiences: Depending on your advertising goals, you can choose the audience that best matches your customers. To drive brand awareness, use affinity audiences to reach TV-like audiences on a broad scale. To reach as many potential customers as possible with an affinity for a specific product area, you might try adding custom affinity audiences. To reach specific audiences actively shopping for a product or service, use in-market audiences instead.
Read more here: https://support.google.com/adwords/answer/2404191?hl=en