Sorry- “Similar audiences”, is an analogy to Facebook’s custom/lookalike audiences.
It takes your existing remarketing list and builds its own audience based on similar interests. It’s compiled from people browsing the Google Display Network over the past 30 days and can be used to serve both text and image ads on the GDN, but your remarketing list needs at least 500 unique cookies to use the feature.
You should only use lists with closely identical user bases, or Google might have a hard time making a similar audience. The similar list sizes are currently small, so you should take precautions such as adding the matching remarketing list as a negative to only serve fresh users.
You can find similar audiences under the Display tab by going to Ad Targeting ->Interests & Remarketing, or in your Shared Library under Audiences, so if you have an awesome list- give it a shot and let me know your results!