A treasury list is a list of some of your favourite items on etsy all put together to make a theme (colour, season, emotion, be as creative as you can!) When you feature someone on your treasury list, they are likely to check out your shop, choose some of their favourite items from your shop, or maybe "favourite" your whole shop which means that anyone who visits their shop will see your shop as well.
It's not all about promoting though, it's about appreciating other peoples work and featuring your favourite listings in a beautiful way. It takes a bit of time, but I think it's well worth it.
First - explore etsy and pick out your favourite items that fit into whatever theme you like. Choose beautiful pictures that are meaningful to you. Make a note of the "listing#" or just add these items to your favourites for easy access later.
Go to the Etsy home page - click Buy (top left) - go to Explore Treasury - on the right you will see "create a list". Click this and start copying the listing # into the little boxes that say "Listing URL". You can rearrange them in anyway you want when you are done.
People will be notified when you feature their item and you should start to see a little more action happening in your shop.
Hope this helps!