Hi Domantė,
You can think of Commonplace as shop windows for the Internet. It's a way of extending your shop's reach and developing relationships with your buyers. Let me explain.
Once you open a free account, you'll be able to import your shop's items to a Collection in Commonplace. Collections are a way to organize your items. You may need only one, or you may find it convenient to have several. In either case, the first stop for your imported Items is a Collection.
When you Create a Place, you are able to arrange your Items on multiple pages in the Place. When people join your Place they can leave comments on your Items, chat with others that are in the Place, or collect your Items for their own Place. It's viral that way.
So a Place is where you can have deeper interactions with those that like your shop. You can extend your marketing reach by embedding a Place in a website or blog. Either way, the links that are associated with your Items link back to Etsy. Commonplace can help you drive more traffic to your Etsy shop.
So to summarize:
Open a free account
Import your Etsy items to a Collection
Create a Place
Contribute your items to the Place (Don't forget to put a Cover on your Place)
Launch the Place and invite your friends
Let us know how else we can help.
Mike